Some of my user are using this models :
http://www.akaipro.com/mpk61
http://www.m-audio.com/products/en_us/Axiom61.html
http://www.novationmusic.com/products/midi_controller/sl_mkii

there is other options too...

If you have already a midi keyboard, you can add that:
http://www.novationmusic.com/products/midi_controller/launchpad

I personnally use
http://www.studiologic.net/vmk-88.html
+ http://www.apple.com/keyboard/

so I can change the sounds wireless.

I am also waiting for some tablets/slates PC who are coming soon. with a touch screen.

I think there is many options, depending on your taste.

I am also waiting for new products

Let me know

Dan