Astronomy Online is an online portal; see also Sky and Telescope. Other resources: NASA, Chandra (via the Chandra Chronicles), and Universe. SETI@home looks for aliens (download the hunt software), the Encyclopedia of Life is more down-to-Earth.
The Official String Theory website covers the next big thing in theoretical physics. Other sites on string theory are Superstrings, the Elegant Universe, and Ed Witten's homepage. Chaos on the web covers the ex-big thing in theoretical physics. Other sites on chaos are this project, and this lecture. The SFI is the original home of complex systems and chaos research. Others include the NECSI, the CNCS, the CSCC, the CCSR, and Exystence. Quantum mechanics is the ex-ex-big thing in theoretical physics. Other sites on quantum theory are this intro, this guide, and this tutorial.