The Peg Solitaire Army is a problem spun off from a classic recreation, and yet another example of the golden ratio turning up where you least expect it. If you want to look at the game more deeply, George Bell's solitaire pages are the ne plus ultra: There's more about the solitaire army (and variants), ... [more inside]
Visual Patterns. Here are the first few steps. What's the equation?
"Gary Foshee, a collector and designer of puzzles from Issaquah near Seattle walked to the lectern to present his talk. It consisted of the following three sentences: "I have two children. One is a boy born on a Tuesday. What is the probability I have two boys?"" [more inside]
The Sexaholics of Truthteller Planet - yes, it's one of those rotten logic problems, one of many that can be found at Tanya Khovanova’s Math Guide to the MIT Mystery Hunt.
A gathering of puzzles including many old chestnuts but also perhaps one or two you haven't met before.
The Mathemagician and Pied Puzzler (PDF, rough table of contents here) is a collection of puzzles created by members of the Gathering 4 Gardner Foundation, in tribute to the man himself (previously). Also freely available at the G4G site is Puzzle Craft (PDF), by Stewart Coffin. (The Puzzling World of Polyhedral Dissections, also by Coffin, is available here.)
MEFIEach letter corresponds to a number 0-9. The solution is unique. [more inside]
Interactive mathematics miscellany and puzzles, including 75 proofs of the Pythagorean Theorem, an interactive column using Java applets, and eye-opening demonstrations. (Actually, much more.)
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.
Su Doku. Fill in the grid so that every row, every column, and every 3x3 box contains the digits 1 through 9. That's all there is to it. It doesn't sound like much, but it's as addictive as hell. The Times is one publication with a daily puzzle (may be unavailable to overseas readers.) There a tuturial and sample puzzle here (flash).
Nick's Mathematical Puzzles. Something to keep you on your toes and exercise your brain this Friday. [not Flash]
Maths puzzles and more problems. Found whilst searching for the fiendish the Monty Hall Problem. A Tangled Tale, indeed.
"WARNING!!! The puzzles on this site are very difficult, and most require the use of a good spreadsheet program in order to solve them. It will take many hours, perhaps days, to solve each puzzle..."
Cut the Knot. Interactive mathematics miscellany and puzzles.
What Color is My Hat? I [heart] these mathematical conundrums -- simple, easy-to-state, seemingly obvious logic problems that have solutions that completely defy common sense. Here's another you can spring on a friend: "You want to fry up three pieces of french toast. You have a frying pan that is just large enough to accomodate two pieces of bread at a time. If it takes you 30 seconds to fry one side of bread, and each piece of must be fried on both sides, how long will it take you to cook up three pieces (assuming that the act of flipping a piece or adding/ removing it to or from the pan takes no time). Think about it. Answer inside.