Frohmage is definitely one of the "must have" free VST effects. i have used it countless times to brighten, colorize, dirty or whatever sounds that were too clean, too dull or whatever. Frohmage kicks ass. The funky GUI may take you a bit to figure out, but after i did, i decided that the funky UI is part of my love of Frohmage.
Frohmage has enabled me to maintain cheesy tracker module instrumentation ported to Sonar by giving them a sparkle (or a grunge) that they lack (module trackers never give the sample back to you as you put it in, i don't know why).
My only negative comment is that it is a mono-effect and therefore you will lose any stereo imaging passed through it. This doesn't matter most of the time (like when i Frohmaged the cheesy guitar licks in an old module), but if you want to maintain the stereo image... i guess you have to go buy QuadFrohmage (which i am seriously considering at this moment, myself).
Frohmage has been FUN and i plan to have more fun with it in future.