Practically all of the symbolic computation done by Mathics uses Sympy. It depends what your interests are. If you're interested in Symbolic computation I'd suggest Sympy. It's a really nice pure python package and it's easy to get started.

If you're familiar with / interested in Mathematica, I'd suggest helping Mathics out. The Mathematica language is massive, and there's heaps of low hanging fruit left in Mathics.

(Mathics core developer here btw)

What are your goals and strengths? I am not familiar with the state of most of those projects, but fixing bugs and completing todo items is welcome anywhere, I imagine, and if your goal is to improve free software then any of those seem a good choice. That's not a very exciting answer, though.

I suggested Mathics because you appear to be interested in what Mathematica can do but resent that it closed source. Since it does not appear that it will be open sourced soon (Taliesin discussed this elsewhere and I got a similar answer from Stephen in an AMA http://www.reddit.com/r/IAmA/comments/tmutz/stephen_wolfram_...), and because duplicating even a portion of a system with a 25-year head start would require a massive amount of effort, it seems the best choice would be to work on a program that would best organize the efforts of the other programs. To agree with that statement one probably has to agree that the language is the best one to build a system around.

Maxima is pretty simple and straight forward (and is kind of a free version of TI Interactive), but if it has to also look pretty, then OpenAxiom is way better as it easily exports to TeX or MathML:

)set output tex on

)set output mathml on