umm... downlaod a crack version of mupad and scientific workplace, MuPad and Scientific workplace are build on LaTeX,
http://en.wikipedia.org/wiki/LaTeX , if you're a tech savvy, maybe you can play around with LaTeX and get somewhere yourself, i'm not a tech savvy so i wouldn't know.
The two programs work integrated with eachother and can be used for writing scientific documents easily (Scientific workplace haivng a "word-like" interface, and writing in math vs text works by clicking a button (similar to MathCAD if you're familiar with that)). They can also display graphs, 3d and 2d, which honestly looks pretty nifty.
but if you're strict on it being open source, then i can't help you, maybe see if there is some smaller project based on LaTeX somewhere? since LaTeX seems to be the preferred coding-stuff for this kind of thing.