The STIX mathematical fonts are available as a public beta. From the website:
The mission of the Scientific and Technical Information Exchange (STIX) font creation project is the preparation of a comprehensive set of fonts that serve the scientific and engineering community in the process from manuscript creation through final publication, both in electronic and print formats. Toward this purpose, the STIX fonts will be made available, under royalty-free license, to anyone, including publishers, software developers, scientists, students, and the general public.
I’m not sure if these work on all platforms, but they are TrueType fonts.
These fonts can be used to augment the symbols available in the OpenOffice.org equation editor (see this tutorial for more).
