Like it's been mentioned already, they all used what we're using now.
Porter Robinson still uses an M-Audio Fast Track Pro
I've seen pics of Deadmau5 on tour with an Apogee Ensemble (He's upgraded to a Prism Orpheus like mentioned above).
Skrillex used his laptop with the audio running out of the headphone jack into a pair of KRK Rokit's (the right speaker was blown btw). He mostly uses headphones because he's always on the road. (I read on another forum that he's been spotted with a Traktor 2 DJ.
EDIT: In issue 253 of Future Music Skrillex said that he's using a Focusrite Saffire Pro 14 and a Shure SM58 for vocals.
Richie Hawtin uses Native Instruments interfaces. I think he's sponsored by them.
Avicii uses an RME Fireface UC.
etc, etc, etc...
The most important thing when picking an interface is doing your research and making sure it's drivers are reliable. Anything below $600 will probably have more or less the same converters, and unless your room is treated and you have really good monitors the more expensive ones won't matter.
And the beautiful mastering you hear is because the tracks are sent to... wait for it... a professional who's only job is to master tracks.