"Open Source 'Sage' Takes Aim at High End Math Software", from Slashdot. (The actual site, sagemath.org, is Slashdotted right now.) Here's the article that Slashdot links to (by the way, the links in that article go to malformed URLs, but they're not hard to fix). The leader of the Sage project is William Stein, who with David Joyner wrote an opinion piece in November's Notices stating that mathematical software should be open source. From what I can tell, SAGE includes both new code written specifically for the project and a lot of old code that's been previously used (it appears to incorporate Maxima, GAP, etc.)
I agree with this; although it may very rarely be practical to check all the work that goes into establishing a mathematical result, it should always be at least theoretically possible. It seems acceptable for the internal workings of mathematical software to be closed when it's just students that are using it, or when the serious researchers are just using it to do experiments. But as we start seeing more and more proofs which reduce to "so I reduced the problem to some finite but very long computation, and then I ran this program, and it said my theorem is true!", it becomes more important that someone can actually verify that the program does what the author claims it does. For comparison, see Wolfram on why you don't need to know what's going on inside Mathematica, which is mildly amusing.
I haven't actually used SAGE, but it certainly seems more in line with the open-information ethos of the mathematical community than the proprietary software that's out there. It's a shame I know Maple pretty well, because that creates a disincentive for me to switch if I wanted to.