Advent of Code is a series of small programming puzzles for a variety of skill levels. They are self-contained and are just as appropriate for an expert who wants to stay sharp as they are for a beginner who is just learning to code. Each puzzle calls upon different skills and has two parts that build on a theme.
Someone in ##programming on freenode mentioned this earlier today, and I though I'd give it a look.
So far, the challenges are pretty simple, but also quite fun.
Also, by solving them you're helping Santa bring us a white Christmas.
Santa was hoping for a white Christmas, but his weather machine's "snow" function is powered by stars, and he's fresh out! To save Christmas, he needs you to collect fifty stars by December 25th.
Collect stars by helping Santa solve puzzles. Two puzzles will be made available on each day in the advent calendar; the second puzzle is unlocked when you complete the first. Each puzzle grants one star.