DANCE! is in open beta. It's DDR Online developed by the guy who made Earthworm Jim. Supposedly you can play with either a keyboard or a dancepad, but I don't have a dancepad and I'm having lots of fun using the keyboard.
So far it's the most positive MMO experience I've had. You instantly feel like you belong to the community because you get invited to a dance the instant you log in. Especially if you're lvl 1 because everybody thinks they can beat you. But Level ultimately has nothing to do with your sense of rhythm, and in my first few matches I beat people a few levels higher than me and got booted from a few rooms possibly for being too good. It feels a bit like Gunbound in that respect, with everybody looking for people that are weaker than they are and trying to avoid the better players. But when you find people who are close to your ability and who are nice, it's really fun.
You would think lag would be a major problem for something like this, but it mostly hasn't been from what I've seen. I still blame it for most of my losses today, but if it's doing this well during open beta I imagine it will be even better when they finally release it commercially.
The best part is the music-- Acclaim got a deal with Warner Music, so they have the ability to license some actual star power for this one. My favorites right now are Missy Elliot's "Lose Control" and INXS's "I Need You Tonight," but they also got that annoying "Ride the Train" song that played at every school dance I went to since middle school. Oh, and Madonna's "Papa Don't Preach" is fun too. These aren't re-recordings like in all previous music games. They actually got the original recordings!