I'm a massive fan of Highlyliquid and his DIY MIDI kits. I own a few, including 2 MIDI'd SK-1's and an MSA that I use on my DR-110. They are bomb proof, people do interesting things with them, and Jon is a top bloke, who actively supports diy communities with lots of giveaways. I won one of his kits in the circuitbending challenge.
However this came out today. Its Tiny (1.04" x .4"), is pre built, has lots of nice MIDI features, has 12 possible outputs and the real deal is that its only $36USD, which will pretty soon be about 2pounds ;#
its could be stuffed into anything, but is especially designed for triggering gameboys, allowing you to sync up all those great 3rd party beatbox synth progs like pounder and muddy gb
*EDIT* too good to be true, from their FAQ;
Do you ship internationally?
A: No. We did for a while and plan on doing it again in the future, but at this time we can only ship within the US.
i'll send them a mail, fingers crossed :#
*EDIT* 'barbie' from division6 got back to me , they will ship to the uk, and offer discounts for bulk orders. who's in?