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.)
posted by cog_nate on Oct 1, 2008 - 9 comments

   MEFI
   META
+ ASKME
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 FILTER

Each letter corresponds to a number 0-9. The solution is unique. [more inside]
posted by Upton O'Good on Sep 3, 2008 - 27 comments

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.)
posted by parudox on Dec 1, 2007 - 11 comments

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.
posted by Wolfdog on May 31, 2007 - 18 comments

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).
posted by salmacis on Dec 10, 2004 - 6 comments

Nick's Mathematical Puzzles. Something to keep you on your toes and exercise your brain this Friday. [not Flash]
posted by Johnny Assay on Oct 1, 2004 - 5 comments

Maths puzzles and more problems. Found whilst searching for the fiendish the Monty Hall Problem. A Tangled Tale, indeed.
posted by plep on Sep 24, 2004 - 6 comments

"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..."
posted by limitedpie on Aug 11, 2004 - 7 comments

Cut the Knot. Interactive mathematics miscellany and puzzles.
posted by plep on Jan 6, 2004 - 8 comments

Fun with Fibonacci numbers. So you say you scored 130 on yesterday's IQ test, did ya?
posted by archimago on Oct 28, 2003 - 5 comments

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.
posted by Shadowkeeper on May 25, 2001 - 24 comments