Comment Re:Slashvertisement (Score 3, Informative) 234
Another vote here for Sage. On the open-source side of things, nothing comes close, because everything else that's any good (Maxima for example) is included within Sage, in a fairly transparent way. (I.e., the user doesn't need to know she's using Maxima.)
Secondly, the (free) support is awesome. If you spend a little while learning Python and the basics of Sage, and you still have questions, the response time at sage-support at googlegroups is incredible.