We have kinda discussed this before:
Apart from the two softwares mentioned there - 'GuitarFX Box' and 'GuitarFX' - there is 'Effects Processor Pro v. 2.2', and also the only free and unlimited but little-featured 'Real Time Effects' (http://www.i-adrian.home.ro/effects.htm). Google search for these and many more programs.
I'll recomend GuitarFX Box.
Many times I've lied, many times I've listened
Many times I've wondered how much there is to know...
Mellow is the man who knows what he's been missing
Many many men can't see the open road...
Many is a word that only leaves you guessing
Guessing 'bout a thing you really ought to know...