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!