A survey of some attractive options:
Livid's controllers are bit higher end, with wooden end cheeks and more robust aluminum construction. They look more like an instrument than your run of the mill control surface. They offer several combinations of knobs, sliders and buttons, including several designs which feature a large central grid of buttons, ala the monome for quick access to your bank of Ableton Live clips (or whatever). They're also into the left/right DJ split style controllers which are useful for programs like Traktor.
Korg's 'Nano' line of controllers probably offer the best bang for the buck. They're cheap and plasticky, and they probably wont endure too many years of consistent use, but at this price point they can simply be replaced. There are three flavors which serve to satisfy your note input, beat input, and knob/slider fiddling needs. There is a discontinued model which offers just 16 knobs; buy ebay cheap.
The Ultranova packs a monotimbral beast of a virtual analog+digital synth into a tight package with a well thought out interface and included Audio+MIDI over USB. Three Oscillators, 2 Filters, 6(!) Envelopes, 3 Lfo's, and a deluxe modulation matrix challenge even the hossest softsynths. The 8 knobs along the top panel function brilliantly: they can be used to edit 8 corresponding parameters on the display, groups of which are selectable with the page buttons below; they can be used to edit 8 parameters on a 'user' page which allows the most important parameters to be gathered on one page for quick tweaking during a performance (ie. no page switching to get to your parameters); finally, they can be touched (they're touch sensitive) to trigger/retrigger any of the 6 Envelopes at will OR activate/deactivate modulations routings established in the modulation matrix. This combination of interface and functionality is all too rare in the world of hardware synths. The Ultranova also includes Novations 'Automap' system which allows the panel controls to be easily mapped to your Vst's, etc.