This list is far from exhaustive. I've only included sites that I thought were interesting and helpful when I visited them. If you have further suggestions, please let me know.

Math texts

Online texts in mathematics

Math World

Lots of great mathematics information, organized by topic.

Planet Math

More good math information, organized by topic. This is more wikilike.

Wikipedia

Whatever its faults (and it has them), wikipedia tends to have very good math and computer science information. Double check it though with another source.

sage

Sage: an interactive symbolic math system. Like Maple and Mathematica but free.

Math Puzzles

Math puzzles and links to puzzles and resources.

More Math Puzzles

More math puzzles (gosh!).

Math forum

A site focussed on learning mathematics (both for students and teachers).

Online Encyclopedia of Integer Sequences

Sometimes when you are working on a problem (or understanding an algorithm) you'll end up with a sequence of integers that represents something about that problem - for example, maybe you are counting the number of ways a specific kind of thing can happen. If so, you might be able to find out something more about that sequence here.