ILIKETRASH  The best thing about SAGE is that there now a $300 version of Mathematica for personal use. ($249 before Christmas, 2009.) I'm really rooting for this software and I tried it in the past but it is plagued by the common open source problem: poor documentation. Also, the interface is clumsy (but what math program doesn't have a clumsy interface--Mathematica's interface was cool in 1987 but it's still pretty much the same). I tried to learn how to do basic stuff using some of the tutorials and manuals but was quickly knee-deep in research mathematics examples. Not good. How many ways are there to make plots? And _every_ math program is deficient unless it uses a mouse-based interface like LiveMath Maker (formerly Theorist) which is still available works after all these years.  
(Version 4.2.1)

CHUCKK  Well if you like linux...

Very typical open source software. Don't even try if you are not comfortable with python. Very powerful but highly user unfriendly, resource hog, does not play well with other software, slow except on a newer machine (lots processors & memory)etc..

Uses a web browser as the GUI, nice idea, mediocre implementation.

But for the price (free) you can't complain to much.

Well at least it was python instead of perl.  
(Version 3.0.5)

MARK EVERITT  Fair enough!

* Firstly it's free (or open source if you like).

* It embeds a useful programming language (python) rather than some proprietary language.

* It includes lots of tools and older computer algebra alternatives such as Maxima and numerical systems like Octave.

* The interface is your web browser! It also does a very nice job of typesetting equations and will even give you the LaTeX code if you ask nicely.

* Plots are interactive. You can even view them in stereo if you have some 3D specs or can go cross-eyed.

* If you like to collaborate then anyone on your network with a browser can join in.

* Much much more!

Ok, I'm done being the salesman now. ;)  
(Version 3.0.1)

MARK EVERITT  I can't possibly recommend this enough!  
(Version 3.0.1)