One thing you can count on for any decently successful rhythm game: someone will figure out a way to make a free PC clone. First there was Stepmania for DDR players, then there was Frets on Fire for Guitar Hero shredders and now there's Osu! for Ouendan/Elite Beat Agents fans.
The public beta, available since yesterday, does a remarkable job of capturing the tap-out-the-rhythm gameplay of the DS games, though the dancing cheerleaders have been replaced with static videos for each song. You can build your own levels or download over 100 "beatmaps" of primarily J-Pop songs that were made by testers.
Casual fans beware -- a lot of these fan-designed levels are killer, though there is an easy mode that makes things a little more manageable. Even if you're a master at the DS games, using the mouse takes a little getting used to. Then again, if you have a tablet PC, it's probably just like the DS games. Except, er, bigger.