KTDrumTrigger is a VST plugin with a custom editor that triggers MIDI notes based on the sound level of the incoming audio stream in different frequency bands. It allows you to "detect" occurrences of percussive sounds in an audio stream and send out a MIDI event whenever that happens.
Incoming (mono) audio is split up into different frequency bands using three tweakable filters. The signal in each of these bands is sent through an envelope follower. When the envelope reaches a specified threshold level, a MIDI note on message is generated. The trigger module has some built-in hysteresis and minimum inter onset time settings to avoid fast successive triggers due to noise on the envelope curves. Visual feedback of the filter and trigger settings is available.