Free PC games for dancing, singing or rocking out
While the PS2 seems to be the platform of choice for mad fans of SingStar, Guitar Hero and Dance Dance Revolution, there are PC alternatives available for free that might give you most of the features without needing to shell out for a console. All of these games are free and open source (source is available on SourceForge).
An open source version of Guitar Hero that runs on Windows, Mac or Linux (and probably other operating systems), Frets On Fire mimics most of the interface and features of its inspiration and also allows you to import the songs if you have the Guitar Hero disc(s). You can also create your own songs or download ones created by others. Playing with the keyboard is a bit off-putting initially and doesn't feel as nice as the slick guitars but you can get used to it – especially if you've got a wireless keyboard and can work out how to hold it like a guitar! You can also plug in a range of controllers if that's more your style.
One of the cool things about Frets On Fire is that it supports community rankings, so you can go online and compete – score-wise – against people all over the world. You can even set up your own private charts server (which uses Django) if you wish.
A copy of Dance Dance Revolution but exceeding the original in some ways, such as having tons of community contributed songs to play with. In particular, if you're a fan of anime theme songs you won't be disappointed. StepMania is available for Windows, Mac and Linux users and is played primarily with the keyboard with the option of adding in your own dance pad for a more authentic (and sweat-inducing) DDR experience. One thing to note with StepMania is that it doesn't include any songs with the application so you have to go and get these yourself. You can find links to plenty of song packs from the game's site but be warned: some of them can be quite large (bigger than 500MB).
In case you hadn't worked it out already, UltraStar is a copy of SingStar, the karaoke game for the PS2. This game is younger than the other two and is understandably less polished but you might still get some fun already. UltraStar is only available for Windows but it appears to be written in SDL and OpenGL so we might see some other platforms available in the future.