VeloScaler is a VST plugin for controlling and modifying the velocity value of incoming MIDI notes.
Use up to 5 different bands, filtered on either the incoming velocity or the MIDI note number. You can set a minimum and maximum value for each band, and scale the velocity by a specified value (either an absolute value, of as a percentage), which can be randomized. This will create interesting variations if your destination instrument is set to respond to velocity.
Freeware for Windows, both 32 and 64-bit.