If you're prepared to use a separate guitar preamp/head and speaker cabinet, there are a couple of Canadian companies produced astonishingly good free guitar amp VSTs.

Acmebargig (www.acmebargig.com) has produced some fantastic amps. The new "Shred" amp is good fun, with a lot of customisation, but I like some of the older "heads", particularly the "Tamla Head", a recreation of a 1960s guitar sound similar to the Funk Brothers Motown sort of sound.

LePou software (http://lepouplugins.blogspot.com/) produces really high quality stuff. The LexTac amp is amazing and completely free.

With all these plugins, you'll also need a cabinet simulator. LePou has a really handy one called LeCab, which is free and dead easy to use. Alternatively, any reverb which loads waves (like Perfect Space in sonar, or Boogex - free from Voxengo) would do. There are literally thousands of guitar cab wav files on the internet. There are some free demo wav files at the Recabinet site (http://recabi.net/site/) which are really cool when used with LeCab.

The separate guitar and cabinet approach is more work, but can be really rewarding when used with any freeware stomp box models.

I also have Amplitube, but that's $159 and not as much fun!

Brian