I'd like to suggest a new freeware plug-in for your consideration.
it's an accurate model of Marshall Guv'nor distortion.
I was going to post it in the free amp sim plug-in thread, but decided I'd better do it here since it's not an amp.
Detailed information is available at the project's homepage:
AXP SoftDrive GV
This thread has some sound clips provided by users:
SoftDrive GV @ UltimateMetal
Since this is freeware, I'll let this go.