Maths Advent Calendar
There is a wonderful maths advent calendar going on at a site called Mscroggs at the moment. Every day the answer is a 3 digit number and the answers will be clues to a murder mystery. Ultimately you are after a weapon, motive, location and the identity of the murderer and the whole thing ends on Christmas Day with prizes for 10 random people with fully correct answers. At the time of writing the first four of the puzzles so far have been well formed and interesting; you can usually do them in about 5 minutes so they are quite nice to wake up to. The fifth one (today's) is proving a little harder and although I have a tentative answer, I need to do some programming to verify it.
Quite apart from the series of puzzles, the whole website is great read of recreational mathematics and puzzles.