A lot of sequencing software like Cakewalk's Sonar/Pro Audio or Steinberg's Cubase SX
provide the ability to use plug-in MIDI effects.
Frank's MIDI Plug-Ins is a continuously growing set of useful plug-ins. Currently seven different plug-ins are available.
Virtual Band automatically generates a complete professional quality arrangement. Just insert the chords using standard chord symbols (like C or Gm7), load the style you would like to use, and Virtual Band does the rest.
Rhythmic Controller Plug-In is a step sequencer for controller events and can be used to automate pan, volume, filter cut-off, resonance and many more.
The Split Poly Plug-In strips a chordal passage into individual single-line melodies on separate midi channels. You could use this feature, for example, to transform a polyphone sax or trumpet section into single-line melodies.
The Humanize Plug-In allows you to vary the pitch, duration, velocity, and timing of notes in your midi tracks. Introducing subtle variations in these paramaters will give your midi tracks an organic feel. No human plays in perfect time, with perfect pitch. The Humanize plugin allows you to emulate and recreate the nuance of human performance via a powerful, easy to use interface.
Delete Doubles Plug-In deletes all kinds of doubled, repeated and possibly unnecessary MIDI events: doubled events, repeated events, octavated notes.
Track and Clip Notes Plug-In allows to have individual text notes for MIDI tracks and clips (if clip effects are supported by the sequencing software).
Midi Statistics This Plug-In analyzes your MIDI events and collects a lot of useful information about Note, Wheel, Patch, Controller, RPN and NRPN events.
Split by Patches is a plug-in for splitting tracks that contain different sounds.