As you will see from the link posted there are loads around.I tried to get some decent sounds using the supplied VSTs but they are very limited in this respect.
The two free ones that are pretty easy to use are Voxengo Boogex,
http://www.voxengo.com/product/boogex/
And ACME Shred(don't let the name put you off it has clean amp sounds as well),
http://www.acmebargig.com/product/shred/
however a few years ago I,"Bit-the-Bullet" and bought a cheap commercial option e.g the Toneport Studio GX which comes with Podfarm amp software among many other great features like zero latency monitoring,
http://line6.com/podstudiogx/
http://line6.com/podstudiogx/sounds.html
It can get the sublimely sweet sound on this track,
http://forum.cockos.com/showthread.p...hlight=lullaby
To the bone crunching sound on this,
http://forum.cockos.com/showthread.p...ghlight=sweaty
All the songs/Guitar/Bass and Vocals in my Reverbnation sig link are done through it as it has patches for Bass/Guitar and Vocals supplied in the Podfarm software.
HTH