Small Midi Keyboard

I picked up an Akai Synthstation 25 for $30 on a clearance sale. It's the same size as my Net Book. 
http://www.akaipro.com/product/synthstation25
I'm not sure if they still make them. 
I then bought an iPhone 4 from a workmate for $50 and installed Garage Band. So I now have a sequencer and / or can play a variety of synths and Pianos live. Lots of freebies for iPhones. 
It also plays just fine into my DAW and Sonar via the USB connection. 
It will run on the batteries or usb buss power. The keys are not full size but not a problem to play for me. 
 

http://www.sweetwater.com/store/detail/microKAIR25
Korg MicroKey AIR

TerraSin
http://www.sweetwater.com/store/detail/microKAIR25
Korg MicroKey AIR

This Korg looks interesting too.  Decisions, decisions.
 
Still looking for other suggestions, too (or experiences with the above.)

I've a Korg Microkey 25. It's actually pretty good in terms of key action, fast and positive. The spacing is tighter than conventional keyboards, but I've smallish hands so the spacing is actually better than a full-size key for me. I'd add that playing piano-style keyboard isn't exactly my first (or even fourth) instrument...

Downsides to it are that pitch/modulation is on a spring-loaded mini joystick which means you can't set the modulation, let go of the stick and have it stay where you left it, unlike an unsprung modulation wheel. It's also very easy to accidentally send unwanted pitch bend or mod unless you're careful to only move the pretty sensitive stick in one axis. No aftertouch, no way to plug in sustain or expression pedals, though on the plus side it has a sustain button and an interesting built-in stick-controlled arp.

Overall though, not bad at all. Works very well with iOS if that matters.

The other small controllers I have are a KMI Qunexus and a CME X-Key 37.

The Qunexus is in a league of its own for possibilities and frustration. It has pads rather than keys, laid out like a 25 note keyboard. Each key can send note on/off and is sensitive to pressure and "tilt" (essentially how you rock your finger forwards or backwards on the key). No dedicated modulation controller and a "tilt sensor" pad that controls (after a fashion) pitch bend. By default it also has pitch bend set to key tilt, and pressure to modulation (CC1).

It can take a standard expression pedal (KMI do a dedicated cable kit, but a cheap 3.5mm to 6.25mm TRS jack adaptor allows an expression pedal to be connected). Also handles CV ins and outs plus there's an optional box to let it talk via MIDI sockets if you need it to. Can send polyphonic aftertouch and polyphonic pitch bend if you need those things.

The pads are very sensitive and predictable for velocity and pressure, though the tilt function is less controllable. Out of the box it was a bit like playing a drunk piano crossed with a synth with malfunctioning pitch bend. The software editor is very good though, and after two or three days of thinking it through and experimenting I've got it set up and configured OK for me. I ended up with key tilt sending a very limited bend range kind of like a guitar bend and pressure sending channel aftertouch. An expression pedal then substitutes for a mod wheel.

It's extremely expressive and I find it easy to play, but piano-players might find it horrible. It's also impossible to play a few chords with conventional fingering because the "white keys" don't have the bit that usually runs between the "black keys". F#minor or Ebmajor are the two main problems.

There's a less expensive more basic version called the K-board that lacks the CV/pedal stuff and has a much more basic editor.

The X-key is more like a "standard" keyboard, almost full-sized keys but with very limited travel - think of an Apple or high quality PC keyboard, that kind of thing. Like the Qunexus it can send polyphonic aftertouch if you need it. The keys are responsive and I find it fine to play, but again someone used to piano-key travel and feel probably won't like it. The difference between the 25 and 37 key models, besides the extra octave, is the 37 comes with the ability to send MIDI over 5-pin cable and connect an expression and sustain pedals.

Other than that, not much to say other than it works as advertised.

The Qunexus and CME boards are both pretty tough and should stand a lot of use, nothing like e.g. the Korg nano-series board's "key" switches.