http://www.stepmania.com/Best DDR emulator, hands down... It works on windows, mac, and linux, and it's way easy to use... the only problem is that it doesn't come with any songs, but if you just bittorrent "stepmania" you'll be fine. I just finished downloading a 5.6gig torrent that has every single official ddr/pump it up/dancing stage version... plus 1000s of other songs... its pretty neat.
I play for an hour or two a day... it's my workout. Ultramix 2 for Xbox has a great workout mode that counts how many calories you've burned, miles you've jogged, etc. It's great for geeks who dont wanna workout, but are starting to get a little paunchy!