What with these kids today carrying around their laptops for DJing, live visuals and VJing, and live laptop music, there's a surprising dearth of physical controls for playing laptops live.
The Behringer BCD2000 B-Control Deejay would seem to cover all the bases on the cheap (US$200 street). For DJs and laptop music, there are 4 channels of audio connected to your computer via USB, with 2 phono preamps, headphone out, and mic in. (So, yes, you can do that whole "two turntables and a microphone" thing.) But this isn't just a DJ mixer: every knob, button, and fader sends MIDI, so you can use it to control whatever music or visual software you like. There are even jog wheels for simple scratching.
Australian VJ Jaymis Loveday reviews the gear, and concludes that it fits the bill, as a cheap entry in digital DJing or a do-all controller for music and visuals. The bad news: MIDI assignments are fixed, meaning some software will need some tricky workarounds.
BCD200 Review [Create Digital Music]