I prefer
www.stepmania.com myself. It actually downloads to your PC and you can load it up with whatever songs you can find. You'd be surprised how many people make stepfiles in their spare time.
www.bemanistyle.com offers plenty of step files to run with it.
EggmaniMN, Reach and Sheepdude release some great step files (some of the best creators I've seen out there I can name off hand)...but I'd reccomend those only for those of very high skill. They're made specifically to be played on the keyboard and far beyond defy the limits of normal DDR step charts. Streams of 32nd notes at 200+ BPM anyone?