Can you suggest a free mathematical software for Ubuntu?

I've already looked at Comparison of computer algebra systems - Wikipedia, the free encyclopedia, but I don't feel like downloading all free softwares and try them all to choose only one.
I'd like to know if you have any experience with one of them that you could suggest me.
I'm using Ubuntu (Linux I guess) 9.10 if it matters.
Also, what is a library? More precisely, what is GNU Multi-Precision Library - Wikipedia, the free encyclopedia Is it a mathematical calculator software? If so... why doesn't it appear in the list above?
Thanks.

P.S.: I'm looking for something that can calculate integrals/derivatives and plotting. Ah, and different form wolframalpha!

I read alot of good things about sage a while ago - had a quick play with it following a few tutorials online and it seemed like a pretty handy piece of software but I had no real need to use anything apart from MATLAB or Python so I cant give a comprehensive review.

Sage (mathematics software) - Wikipedia, the free encyclopedia

As for a definition of the GNU Multi-Precision Library, I would describe it loosly as an "addin" that you can use in whatever computing language you are working with to support number types where regular interger and double precision numbers are not big/accurate enough etc. It is not a standalone program.

Elbarto

Thank you very much elbarto! I'm downloading Sage.
I'm still open for other ideas.

No problem. One other neat thing about sage is that you can sign up and use it online here: Sign in

Obviously it wont be as fast as something running on a native machine but it is a pretty impressive feature for the price (free).

Elbarto