The eyeballing game: compare your best attempts at several instinctive everyday tasks - determining a point of convergence, bisecting an angle, finding the midpoint of a line - against mathematical certainty. In a more financial mood? Play Chartgame: given a random historical stock chart of an unnamed S&P 500 company, choose to buy and sell as time advances to see if you can beat the market.
SlitherLink - a little spatial-numerical puzzle. Here's a better exposition of the rules from the puzzle's inventors, and another collection of puzzles. Oh, and a little survey of other sneaky, snaky puzzles.
The Pianolina - an addictive flash game - is something like a cross between Pong and WolframTones. Brought to you by Grotrian, piano manufacturers since 1835, the pianolina visualizes musical notes as little squares that chime when they bounce against each other or against a wall. Its sophisticated interface lets you add chords, gravity, or start with the basic notes of well known compositions like Beethoven's "Für Elise".
Flash Animations for Physics. Animations and interactive demos available in many varieties, such as classical mechanics, nuclear, quantum, and relativistic. There's even a nice explanation of the forces at work in Curling. And if that doesn't wet your geek whistle, then take a peek at the patterns of Visual Math.
Friday Flash Fun, and other games from the
educational entertaining Maths is Fun. Just don't tell the kids they're learning.