Kenton Pro Solo MIDI/CV interface.
This came with the Analogue Systems modular and it's wicked. Before this I had always used multi-channel devices which were limited in that sometimes you can only assign certain functions globally over all the synths you are controlling. Because this is a single channel device I can use all its features. It's very cool for doing portamento, using MIDI controllers to affect different paramaters on the modular, and the built-in LFO occassionally comes in handy.
Kenton website here
Sound On Sound review here