There are many VST plugins for guitar amp-cabinet-stomp boxes simulation,and many of them are excelent.But i found an set of plugins that are free and even doesn't have a user interface but sounds great,i would say absolutely in range of high cost commercial products as amplitube,or native instruments guitar rig,or waves guitar rack.

With some nice graphical interface they would be hit im sure.Realy don't understand why that suite is free:

www.simulanalog.org/guitarsuite.htm