I've been running a little physics forum, where the categories are as follows:

Time and Space
Energy and Mass
Light and Electromagnetism
Quantum Mechanics and Particles
Cosmology and the Universe
Physics Research _{Tell us about your work or other work of interest}
Physics Politics, Funding, and News
Forum rules&terms of use, and other forum matters
Other

I'm not quite happy with the above, but it's surprisingly tough to come up with suitable categories. It's going to be even tougher when it's the whole of science rather than just physics.