This one:
Behringer Ucontrol
It's $99, which is as cheap as they get.
It gives you 25 keys (with octave switching), 10 programmable controls (which will allow you to map out particular controls you frequently use on your synth, like cutoff, etc), 8 programmable buttons to switch between setups for inter-synth use, plus 50 virtual instruments to add to your collection.
Honestly, this is a good bargain. You'll be able to have tactile control of your virtual synths, plus add the 50 that comes with the keyboard, you'll have everything you need for recording or performing live.