I've been using the M-Audio Evolution ones for a few years. They are built like Bic-lighters but they have lots of features and are light for gigs. I'm using an MK-461C.
For me, I need big mod/pitch wheels and 61 notes is what I prefer. I don't need weighted keys, I actually like the plastic crappy feel of the Evolution keyboards - it's ridiculously fast and can do very short notes.
I had to buy a few keys that broke off, and I ended up buying an old one for parts, but I probably use mine more than most people would.
I use the Mac freebie MidiPipe to set the parameters that my Evolution operates at. It gives much better control than the keyboards onboard controls.
I always buy used off of Craigslist or eBay if I can. If you're not particular, any ol' keyboard with MIDI will work.