^that^
If you're wanting something that will create a song from tab, then TuxGuitar will do that. It's free.
.:: TuxGuitar ::. Open Source Tablature Editor :: TuxGuitar
As far as something that will just create a song based on some parameters, then there's Jamtrax:
Sony Creative Software - Jam Trax
Other than that, Tuomas pretty much summed it up nicely. Here's a Free VST player, and some VST plugins to get ya going:
VSTHost Free Download - Open VST
VST 4 FREE - Guitar & Amp Plug-ins