I’m a big fan of puzzles in roleplaying, but it can be hard to come up with puzzles sometimes as a DM. There is also the debate about how much you should support players in solving the puzzle, and if a low intelligence PC can still solve these types of puzzles (at least using something other than a big ax or hammer, of course).
My usual rule of thumb, anyone can help solve the problem. Players can use skills to get clues about the puzzle, but I like the idea of rewarding players with more when they don’t use clues.
Here are some resources for using puzzles in RPGs.
Tutorials and Suggestions
Preference is Puzzling: a great post suggest ways to incorporate puzzles into D&D and suggestions on how to deal with player vs. character ability and knowledge.
Runes for Puzzles: The runes can represent numbers or anything else you might need them for in a puzzle or riddle.
Mathematical Puzzles in Fantasy Games: Some great mathematical puzzles that can be used in fantasy games.
Braingle: Brain Teasers, Riddles, Trivia, Brain Exercises and Games that can be adapted for your game.
Cloud Kingdom Games: Publisher of Riddles and Puzzles.
The Ultimate Puzzle Site: Tons of puzzles and riddles.
Enworld Discussion of Puzzles: This discussion has some great ideas for puzzles like a monster that sucks in all attacks against it. What are the players going to do?