It seems like most people around here use the Teensy 2.0 to run their keyboards. It's got 25 pins, which would be enough for basically any keyboard, and it's nice and small. The only issue is that it's a little pricier than I'd like at $16 a pop. (Yes, I'm a cheap-ass.)
The main alternative I refer to is the "Arduino Pro Micro" clones on eBay. Half the cost of a Teensy will get you the same processor, nearly the same dimensions, but only 16 pins -- not really enough to run a decently-sized keyboard.
Then there are some other interesting bits of hardware out there. $20 will get you a Teensy 3.1, with 34 pins and a 32-bit processor clocked at 72mhz. In other words, enough pins for an F-122 without breaking a sweat, and enough processing power to run a 3D printer.
Last but far from least, there's the Arduino Due. $22 buys you enough pins and power to run, by my estimation, three decent-sized keyboards. A larger package, yes, but still quite promising.
Unfortunately, the latter two both use processors which wouldn't be usable with current firmwares that I've heard of without a major rewrite. Are there any usable firmwares for 32-bit MCUs out there? Are there any options I'm missing?