Jonas Norberg has released Sniper, a freeware VST plug-in which contains 64 narrow notch filters controlled by MIDI.
By default no filters are active. When a note-on is received a group of notch filters corresponding to the harmonic content of the note is started.
In special circumstances this plug-in allows you to remove or attenuate specific instruments from a mix while leaving the other virtually untouched.
Check Jonas Norberg’s page for more information and a link to download Sniper.