Searches a txt for swear words and replaces them with euphemisms of the swear word
Directory walker - list all files and directories.
Graphics tech demo including procedural terrain generation and post-processing effects, with A* path-finding.
Write a genetic algorithm.
Write a markov-chain IRC/IM bot.
Hanoi tower solver in 5 lines or less.
A 45 byte executable file (Yes, it's possible)
Random assembly challenges.
Memory management module, including a static 2-4mb buffer to 'allocate' out of, with a background thread acting as a memory de-fragmenting agent.
Project Euler.
Write bindings for a bunch of scripting languages such as Python and Lua to a game (new simple game, or existing game with SDK available)