A mate of mine at work has just asked me the same question (Hello Carl, if you're reading this).
I'm going to point him towards this thread, but here's the text of an email I've just sent him;
These one's are all set up to use Traktor's internal mixer;
Vestax VCI 100 - seems to be the controller of choice for a lot of people at the moment
M audio torq xponent - Also includes soundcard. It's what I currently use, & I'm very happy with it. Only bad point possibly is it's quite large
Numark Total control - similar to Vestax & Reasonably priced. I owned one for a while & really liked it.
Numark - Omni control - New controller from Numark. Similar to Total control, but now has better build quality & includes internal soundcard
Hercules - Cheap copy of the Vestax, but a lot of people on the forums who own them seem very happy with them. They do two versions, one with & one without a soundcard built in
Reloop - Seems to be a copy of the Numark total Control. Again, two versions, ....with or without soundcard.
M audio - X-Session Pro - Traditional mixer layout, & fairly cheap to buy. Also includes a free copy of Torq LE. I suspect that you'd soon run out of assignable buttons if you used it with Traktor
Vestax VCM 100 - Slightly odd layout, as it uses rotary faders. does have onboard soundcard though.
Vestax VCI 300 - Still a bit of an unknown at the moment. Designed to work with Serato's "Itch" software, but should be ok with Traktor (once they get the Hi-res midi problem sorted). Onboard soundcard, & excellent build quality.
Fader Fox - I still own one of these, & they're excellent! Great if you're tight for space, but possibly short on buttons (like the X Session Pro). Uses standard midi cables rather than USB, but that's not a problem if your soundcard includes a midi interface (most of them do)
If you're using an external mixer
Stanton DaScratch - ask me in a few weeks time, after I've brought one!
EKS Controllers. - Not sure about the new big "Otus", but a lot of people swear by the XP5's & XP10's. ......soundcard included as well.
Allen & Heath Zone 1D & zone 2D - Serious kit! ....the 2D has the soundcard built in.
Any other midi controller in the world. - Why not? you can configure your existing keyboard exactly how you like. I also own an M-audio Trigger Finger which I've used for DJing on several occasions.
This site & it's forums are worth a read. Lots of good tips & comparison stuff.
also, get yourself on the proper Traktor forums