AudioMulch 2.2 is just around the corner. It adds MIDI output contraptions. This makes plugins capable of processing or sending MIDI much more interesting to use in AudioMulch, since you will be able to send MIDI from plugins to outboard MIDI hardware.
We've put together a list of MIDI processing and generating plugins for Windows below. Please let us know about any others and we'll update the list. There's also a related list of Mac plugins here.
[Last update: July 20]
Piz pizmidi collection - small useful MIDI processing plugins. Currently free and highly recommended.
Piz MIDI plugins - additional MIDI plugins from Insert Piz Here
ReaJS and ReaControlMIDI - scriptable effects and flexible MIDI controls
"It already has a ton of effects/generators presets including MIDI processing plugs."
UplandToys BallSequencer 1.5
XOXOS MIDI plugins (Freeware, Windows) - Series and Populus are recommended.
Eckel - Euclidian sequencer
Cableguys MidiShaper - flexible LFOs and envelopes
MIDIplugins.com database (last updated 2008, Windows only)