This is a great online equation editor. You enter a Latex string (with helper buttons to help you build the equation). Then you can download image to your web site. Amazing!

There are only 5 billion essentially distinct Sudoku puzzles (solutions) (proof here using Group Theory and Burnside's Lemma).

Interesting article on rating the difficulty of Sudoku's and various solving methods (including a short backtracking program written in Ruby).

Really interesting problem in recreational mathematics - try to reverse a unit line segment via translations/rotations - but in doing so sweep out a minimal area.

PI/4 is the obvious minimum - but it turns out you can do so in an arbitrarily small area.

What's odd to me, is that any combinations of moves that are either a) rotations about the center and b) translations, CAN NOT achieve an area less than PI/4; so it's counter-intuitive that you can do better than that.

Really insightful article about the difficulties of writing correct software - particularly in the realm of numerical calculation.

Also references an Excel 2007 calculation (display) error where the formula 850*77.1 is displayed as 100,000 (instead of 65,535)!

refaving darrellp

PDF files for various polyhedra you can print, cut, and fold.

Interesting page on non-periodic tilings, including Penrose tiles.