I've collected a fair few over the years:
Keystation Mini32 : Probably my favourite for portability, with good velocity curves, but no extra assignable controls for synths etc.
MidiTech i2-mini 32 : Standard mini keys (as opposed to the micro keys found on the keystation). Velocity curves are a bit harsh for my liking (it's either really quiet or really loud). Real pitchbend & mod wheels + input for sustain pedal.
Korg MicroKey : Good action on these. I've got the 61 key version, but they do smaller ones (25, 37 & 49). No assignable controllers tho. I suspect the 25 key or 37 key version would be a better choice than the Keystation Mini 32. Korg NanoKey is another alternative, but the keys look odd to me. Again, no extra controls.
EMU X-Board 25: This one has full size semi-weighted keys with aftertouch, and 16 rotary encoders. I've also got the 49 key version, which is my 2nd to main studio controller (my main one being a SL990 88 key weighted keyboard). I use the 25 key for programming sounds. This controller is fantastic in every way. It's not that small though (about the size of a laptop). No longer made, but they do appear time to time on ebay.
Its worth mentioning that out of the lot, only the Keystation Mini-32 (or Korg NanoKey) will actually fit into a laptop bag along with the laptop.