Sloane's On-Line Encyclopedia of Integer Sequences will give you the answer for any sequence type problem, such as (What is next in this sequence: 1 1 1 3 12 24 66 ??). An excellent site (and book).
Favorite Mathematical Constants by Steven Finch contains lots of details about any mathematical constant. If you need a few dozen different ways to generate Catalan's constant, you can find it here.
The Inverse Symbolic Calculator allows you to put in a number, such as 0.460019114058501360975273285624, and the online program will search through its database of several million entries to try to find it. Its smart lookup function found my number (1/7 + sqrt(17) / 13) in a few seconds.
Interactive Mathematics Miscellany and Puzzles, or cut-the-knot, is a site by Alexander Bogomolny devoted to math puzzles! A fascinating site.
The CRC Encyclopedia of Mathematics by Eric Weisstein contains over 9000 mathematical topics.
The Math Forum Internet Resource Collection contains many links to math sites, with a brief explanation to allow looking before leaping.
The Geometry Junkyard by David Eppstein contains hundreds of excellent links to the more visual portion of mathematics.
CALCULATORS ON-LINE CENTER is an incredible collection of thousands of calculators put together by Jim Martindale. If you need any type of calculator, you can find it here.
Ask Dr. Math contains many answers to questions about math. For a K-12 problem, this is a great place to start.
The History of Mathematics site by David E. Joyce has math history. Elsewhere, he talks about wallpaper groups.
Ivars Peterson's Mathland contains many nice recreational math articles.
Euclid's Elements has been rendered in stunning detail by D. E. Joyce. Each geometrical property comes complete with diagrams and Java applets.
KnotPlot by Robert Scharein contains many beautiful pictures of complex knots.
Dr. Matrix maintains a list of excellent sites in Mathematics. Many of the sites listed in this site were brought to my attention from the list at the Dr. Matrix site.
David Singmaster has an excellent list of mathematical recreations resources. This is the list that gave me the most help on my thesis.
The Optimal Golomb Ruler website contains answers to questions of the following sort: Can you find the lowest set of 8 numbers such that the differences between any two of them are all distinct?
At Mauna Loa, yearly
concentrations of carbon dioxide have been graphed. Take a look
at the graph.