There’s a lot of options posted already below: but in case you’re interested in DIY electronics (I know not all are…) its relatively easy to get MIDI output from an Arduino, and use it with a MOD!
The lovely folks over at LMP even have a tutorial for building a sequcener:
The Arduino tutorial is good for understanding the basics:
I think the power of using an Arduino and building your own MIDI controller is huge - total control* over button layout, functionality and FX
*some electronics / programming effort required