I haven't tried any of them, but a quick google search turned up Steinberg's Virtual Guitarist and Cutter's Revitar. Neither are free, but there may be trial versions that you can check out before buying.
RealGuitar is a decent one for acoustic but I really haven't heard any guitar VSTi's that sounded "good" or something that I would want to put in one of my songs.
I would check it out at least. They have samples on their site.
Free? Use soundfonts (you can use them with any soundcard).
Go to www.rgcaudio.com and download their free SFZ soundfont player.
Read the directions on how to install it and point your midi tracks at it.
Then go to www.hammersound.net and find some guitar soundfonts.
No it won't replace lead players with talent, but it is free...
Izotope Trash. I have that one.. with a ton of plugins. I don't really like using them though.. they don't have a great sound, but decent. I prefer just micing an amp or using a digital effects pedal.