Lately I came accross this project while seeking an open source alternative for Mathematica and Maple. Here's the official site. The project started in 2005 and has been actively developed until now. The newest release is 3.4, which came out just more than a week ago.
SAGE is essentially a collection of different softwares. It combines many math packages like PARI, Maxima, etc. and make them work together seamlessly.
I found this program powerful and very easy to use. It's written mainly in Python, and you interact with it using Python's syntax. This is a huge advantage over using many packages separately, since one would have to learn each separate program's syntax then.
The software comes in two flavors: a standalone desktop application, whose source code is around 200MB (I had to compile it from source because there were no pre-built package for my Linux distro), or you can sign up to use the web application flavor. So far I have not seen any difference in functionalities between the two, but I believe they mention that somewhere in their documentation.
One can get used to the program quickly by following the tutorial. Last month I wrote a calculus library using Haskell. This software makes my work looks like crap, what a shame :)
