LePou and Amplitube are some of the clear top guns. Having worked with both, I wouldn't say either is better or worse, but certainly different. LePou is wayyyyy lighter on your cpu whereas Amplitube makes my setup sound like a microwave. Plus the credits system on amplitube is no fun.
One of my favorite free virtual guitar amps is NadIR from ignite amps. Whenever free also joins something I like to call 'not shitty', that's what I call a win-win