The Korg MicroKEY25 USB MIDI keyboard is a good "portable" tool due to its small size. It is USB powered and equipped with a joystick for modulation/pitch bend, a key to activate the arpeggiator (which is tempo-synced), a key for sustain, and of course the keys for changing octaves.
The keys are semi-balanced ("Natural Touch") and give a good feeling.
Compatible with Windows, Mac OS X (as well as iPad), I tested it with FL Studio without any problems.
Recommended, especially considering the price.