Pzzlr is a great, FREE site that will get your brain thinking.  Pzzlr has many puzzles, brainteasers and logic problems that are fun to try and solve.  The site does have ads on it and depending on those ads, plus the age of your students, you may just want to print the problems off yourself.  A nice thing about solving each problem is the “Continue to Solution” button.  Here you will get and answer and some explanation around that answer.  So, the next time you need a question to stump your students or just want to challenge yourself, check out pzzlr.