REDU(P)CER is a free VST Effect plugin from Morfiki. It is a bit reduction effect using a very cool control set up that allows you to morph between five presets at a time. By assigning a midi controller you can get some really interesting automation effects in realtime. The effect wiorks really well and is simple and fun to use. [...]
Livecut is a free VST effect plugin by MDSP that slices and glitches the audio being fed to it on the fly. Easy to use and a nice gui Livecut VST can take a boring track and add that extra touch.
Fretted Synths Audio has a free vst effect plugin called Free Amp which is an amplifier and speaker simulator with effects. Great for use with live instrument line feeds especially guitar and bass but will work well with other audio sources as well. You have a choice of speaker boxes and microphone placement. You can display the effects as stomp boxes or as rack units. [...]
Glitch is one of the more interesting free VST effect plugins available. Glitch from DBlue (Kieran Foster) is a good multi-effect processor with some unique features. DBlue Glitch is a multi effect sequencer. The incoming audio gets divided into individual steps of various lengths. A variety of effects can then be applied and mixed into those segments and the sequence looped.