Advent of Code (AOC) is crated by Eric Wastl and is an annual advent calendar of small programming puzzles. They can be solved in any language (you don't really need code at all to solve some of them) and its intended for people of all skill levels. There are a total of 25 exercises per year, one per day starting on December 1st and ending on December 25th (Christmas).
You can find all of the advent of code exercises on the official website here: