First off, these aren’t programming challenges in the classic sense. Second, you do real work. So, get to work!

Matsano Crypto Challenges has you write attacks on cryptographic algorithms and systems. It requires no pre-existing crypto knowledge, and a very moderate level of math (about 9th grade level).

The Eudyptula Challenge has you work on the Linux kernel, including having patches accepted. Think of it as a guide to “how can I contribute to the Linux kernel?”, or, really, any large open-source project. It does require you to be a fairly strong C programmer.