Some puzzles and brain teasers are not simple exercises - they're bordering on torture. But that's what makes a good puzzle good: it seems impossible and gets you all addicted till you solve it. So now that the days are getting shorter and the weather getting colder (at least for us folks in the Northern Hemisphere...), here are some logic and brain teasers to pass keep you company.

• wu: riddles: the more mainstream kind of logic problems classified as easy, medium, and hard. Prepare to waste hours on this collection.
• Classic Computer Science Puzzles from Coding Horror. If mathematical algorithms are your thing, these should be easy. Ha ha.
• Project Euler is a collection of math problems that require programming to actually solve.
• Finally, the shameless plug: blogSci's very own Daily Mental Calisthenics gives you word of the day, a Stroop effect puzzle and a Sudoku to kick start your day.

Please let me know your favorite collection of puzzles and brain teasers either in the comments below or by email.