The people behind the free and open source computer maths system Sage have opened a new site called interact.sagemath.org, containing interactive demos of things you can do with Sage.
It’s probably a response to the Wolfram Demonstrations Project which showcases similar applets made with Mathematica. The Wolfram site requires you to install the player for their quixotic “CDF” format, while the Sage demos should run in any normal browser. Another plus is that you can edit the code behind the applets inside the page, instead of downloading them to your PC.
At the moment the Sage site seems to be running very slowly so I haven’t managed to actually run any of the demos but I’m sure they’ll sort it out eventually.