Freeamp is OK, it is free.
I use it mainly for bass amp sim, (when I am not using a real bass and sending it through the GNX4 or Johnson station and a Trident/Joe Meek) mostly for midi bass lines that I want to add some punch to.
First I send them through a VST sfz player with a clean sound, then through the Amp sim.
It may work well for guitar, although programming guitar in midi to have a human feel is far more difficult for me than just picking one up and playing so I have not had the need to amp sim a guitar.
you didn't specify what type of guitar you are wanting a software sim for (bass, acoustic 12 string, electric, etc etc) so this may or many not be helpful.
There are some really interesting and usable VST's out there many of them free but for specific guitar instruments.
keep shinin
jerm