Relativity on the world wide web maintained by John Baez, also included under 'science texts' (but worth a double mention).

The Light Cone Rob Salgado's 'illuminating' introduction to relativity.

The Solar System (University of Tennessee, Astronomy 161) the basics, presented in an accessible way (section 6 'The Development of Modern Astronomy' is particularly useful: introductions to Aristotle, Kepler, Galileo, Newton, etc.)  The 'Astronomy 162' site  is also worth looking at.

MacTutor History of Mathematics archive (St Andrews) Accessible introductions to a lot of useful geometry and physics (see 'History Topics: Alphabetical Index')

American Institute of Physics: Exhibits and Online Source Materials for History of Physics and related fields

Astrophysics in Cyberspace - the High Energy Astrophysics Science Research Archive Center

The Evolution of the Concepts of Space and Time Michael Bradie and Comer Duncan (good historical introduction to all the main figures, from antiquity on)

Jeff Weeks' topology games a useful way of gaining an intuitive appreciation of (very simple) multi-connected spaces.

Andrew Hamilton's homepage featuring videos of what you might see if you were to fall into a black hole, a 'tour' of STR (including a web-based relativistic flight simulator), and simulations of four-dimensional geometrical objects.

Warp allows you to visualize relativistic phenomena that are generally unobservable in the everyday world, e.g. changes in appearance that an observer will see as an object reaches very high speed relative to the observer.

Quantum mechanics made simple a ThinkQuest site, an elementary introduction.

Time Travel Steve Preston's site, with useful information and links.

Entropy on the World Wide Web

Loop quantum gravity theory Lee Smolin's site - plentiful links.

Imagine the Universe! - a NASA educational site, the focus is usually on recent discoveries in astronomy (high production values!)

The Search for Extra-Terrestrial Intelligence (SETI) League - self-explanatory