I use a bunch of VSTs, so I thought I should share the wealth (of information) so others can check them out. All of these are FREE.

Simulanalog guitar suite:
JCM900, tubescreamer, DS-1, SD-1, Oberheim PS-1, Univox Univibe, Fender Twin
They all sound good, but I only use the tubescreamer and the JCM900, which sound fantastic.
http://www.simulanalog.org/guitarsuite.htm

Poulin audio:
A lot of people like the Lecto, which is a mesa/boogie VST. I like the Legion, which is an incredible high gain amp with a great tone. He's got plenty of amps on here, all sound great, and also the LeCab, which is a very good speaker cab simulator that uses very little CPU power. The LeCab 2 is even better, but uses more CPU and you have to get impulses for it. I use both cab sims, they're great. Also has some Mac versions.
http://lepouplugins.blogspot.com

And since you need impulses for LeCab 2, here's a shitton of free ones! Also known as "god's cab" impulses.
http://signalsaudio.com/index.php?optio ... &Itemid=15

Dirthead: A mesa-boogie clone, basically. Unlike others though, this one actually sounds like a mesa. The tone knobs are very sensitive, so I advise moving them just a fraction at a time. Also, when you use it there's a little glitch; whenever you load a project file, you have to re-click the channel selection LED on the amp. When it loads, no channel is selected, and each iteration of it uses like 70% of your CPU, so it's pretty nasty. I get around that by leaving it bypassed before I save and close any project files that I'm using it on. When I open the song up, I just un-bypass it, and everything's fine. That small glitch aside, it sounds fantastic. It has a built in cab simulator that's okay, but I don't personally like it. I use the LeCab 2, and the combination is great.
http://www.ndzeit.org/guitar/dirthead.html

Acme Bar Gig - Shred 1.0
A good product, high quality sounds, etc. They have a marshall clone that can do metal sounds, but overall it's more suited to hard rock and less. I've used it on a stoner metal/bluesy hard rock song of mine successfully. Still a great product, and free!
http://www.acmebargig.com/product/shred/
Also by the same company is the Razor:
http://www.acmebargig.com/product/razor/
The series 60, which they say is their heaviest amp
http://www.acmebargig.com/product/series-60/
and the C-15
http://www.acmebargig.com/product/c-15/
They have a lot more, search under "products"

Aradaz amp VST. I use a ton of VSTs, I've probably tried every free one out there, and honestly I don't remember how good this one is. It's still on my computer though, so it can't be bad. Anyways, it's free and there's also Mac versions.
http://aradaz.blogspot.com/p/download.html
They also have a nice cabinet simulator that has 5 or 6 different cabs and allows you to use any combination of them. It sounds really nice.
http://aradaz.blogspot.com/search/label ... 0Simulator

Nick Crow Lab. His 7170 is a 5150 clone, and the 8505 is a 6505 clone. Both sound incredibly good. His tubedriver is a tube preamp, but I've never used it. I probably should try it, as his other stuff is top quality.
http://nickcrowlab.blogspot.com
Also, the simple combo. 3 knobs, very metal, and perfect for those with limited CPUs, as it uses very little.
http://requietus.blogspot.com/2008/11/n ... o-gui.html
The wagner sharp MKII. I haven't tried this one yet, but I will today. I'm sure it will sound great.
http://requietus.blogspot.com/2008/11/n ... i-gui.html

Wurr Audio Green Machine. It's a nice amp sim that pretty much does everything. It has a ton of options, and a ton of gain, and really captures the "marshall roar" kind of sound. It's really a nice sounding VST. This VST really can do about any amp sound you can think of. I've had an issue with it freezing up/crashing sometimes when I use it in Reaper. Still worth looking into though. Also on this page are a tube booster and reverb.
http://www.greenmachine.pwuq.de/Wurr_Au ... nload.html

Ignite amps. The NRR-1 was apparently made for the guitarist of Fleshgod Apocalypse. I haven't tried it yet, but I plan on it. Also has mac versions.
http://www.igniteamps.com/index.php?opt ... 55&lang=en

Bluecat freeware pack. It's a pack of various tools, spectrum analyzer, EQ, gain controller, flanger, chorus, and all very high quality.
http://www.bluecataudio.com/Products/Bu ... ewarePack/

I've tried more, but I didn't think they were good enough to post here.
I'll try to record some quick samples of many of these and post them here over the next day or so.

Well, here's a link. The guitars on this song were recorded with the tubescreamer and JCM900 from the simulanalog guitar suite, and the Poulin LeCab.
http://dl.dropbox.com/u/72840986/blinde ... %20mp3.mp3

Okay, some sound clips. This is me just dicking around on the guitar, mostly so you can hear the tone. The guitar is tuned to C standard, humbucker in the bridge position. Each was recorded with just a single track. The only other effects was just a simple high pass at 100, which is pretty standard when recording metal guitars.

Guitar Suite tubescreamer, Nick Crow 7170, Poulin LeCab:
http://dl.dropbox.com/u/72840986/7170tu ... rlecab.mp3

Dirthead, Poulin Lecab2:
http://dl.dropbox.com/u/72840986/dirtheadlecab2.mp3

Here's a song I recorded for my wife using the above dirthead/LeCab2 combo (lead guitar is the TS/JCM900/LeCab, though)
http://dl.dropbox.com/u/72840986/consum ... adaver.mp3

Poulin LeGion, Poulin LeCab 2
http://dl.dropbox.com/u/72840986/legionlecab2.mp3

Guitar Suite tubescreamer, JCM900, and Poulin LeCab
http://dl.dropbox.com/u/72840986/tubesc ... 0lecab.mp3

Nick Crow Wagner sharp MKII, Aradaz Our Cabinet Sim
http://dl.dropbox.com/u/72840986/wagner ... netsim.mp3


Couple notes. I just tried out the Aradaz Our Cabinet Sim, and I really like it. I'll update my first post to have this link in it as well.
http://aradaz.blogspot.com/search/label ... 0Simulator

The JCM900+tubescreamer combo sounds really nice, but the low end needs to be tamed (many of Poulin's amps are that way as well), and it tends to swallow the bass guitar sound. The Dirthead has a nice mesa-like sound and really lets the bass guitar shine, which is important for getting a big, thick guitar sound.

Reposting my in-depth look at most of the free stuff on the net. I might add a small review of Amplitube later.

LePou
~ LeGion (original amp model from Poulin; it's great for rhythms, but I wouldn't use it for leads)
~ LeCto (supposed to be modeled off a Mesa Boogie Double Rectifier, I think. It does the job great, and has two different drive settings. Both are pretty high gain, so you could probably find something you like out of this)
~ LeXTAC (this amp is creamy sounding, but very high gain. It's awesome for thick leads that cut through the mix)
~ Hybrit (this is a great british Class A amplifier meant to model 70's Marshall sounds. With a Tubescreamer plugin, it can be used to attain a really thick, doomy tone)
~ Le456 (this is an Engl Fireball model. The GUI isn't exactly my favourite aspect of this amp, but it's pretty good at modeling the high gain and the punch of the Fireball)
~ SoloC (this is a Soldano amp model. It's sort of like the LeGion almost, but it's got more versatility in the tones you can get. I haven't played with this one too much, honestly.)

Ignite Amps
~ NRR-1 (If there was ever an amp that should be mandatory to have in your collection of sims, it's this one. It's clean tone is GOD LIKE, and it can only be rivaled by the clean tones I've gotten with Amplitube 3. It's absolutely fantastic. It's got really great drive too, but clean is definitely it's strongest point, hands down.)
~ The Anvil (I think, personally, this is a weaker version of the NRR-1, but it's still got some strengths about it. Haven't used it too much since it's pretty resource heavy.)

TSE
~ TSE X50 (this is a Peavey 5150 amp sim, supposedly, but I think the sound and design is a bit closer to the 6505. Calling it great doesn't do this amp justice; it's probably one of the best free amp sims I've used, and it's got monster gain. My only complaint is that it's incredibly resource heavy compared to other amp sims, so keep that in mind. It's still definitely worth the download.)
~ TSE X30 (this is an ENGL Tube Preamp 530 model. This thing is like the X50 but just a step under it. It's really great, and has really colourful distortion, as well as a great EQ section. Probably still my favourite amp sim.)

Stuff you MUST get
~ TSE 808 (Excellent Tubescreamer 808 sim, and it's fantastic. It's got the tone right, doesn't cut too much low end like other TS plugins, and boosts the signal.)
~ LeCab (this is an impulse loader from LePou. LeCab 2 is also out, but I think LeCab is simpler to use if you're not recording. There are MANY MANY MANY impulses from many different cabs all around the internet. PM me if you want some links)

Other Notable Amp Sims
~ Peavey ReValver mk3 (this is an official Peavey software in which their most famous amps are modeled, along with some generic stomp pedals, and a great option to mod the amps to your liking by changing the tubes, adding tubes, changing tonestacks, etc.)
~ Ronald Passion (first thing you should do is turn off the cabs since they're pretty garbage. Next, get LeCab or LeCab 2 and load some cab impulses of your own. It's great for dirty drive tones, but just get rid of the stock cabs!)
~ Simulanalog Guitarsuite (this comes with a Marshall JCM900 sim, a Fender Twin Reverb sim, a Tubescreamer plugin, and various other famous effects. Don't download it from the official download page. Instead, get it from Requietus)