These are some crackmes. I wrote them to familiarize myself with reverse engineering, and to show off specific concepts and techniques.
I wrote a tutorial about these crackmes as well.
To work with them, run make <name> where <name> is one of crackme01, crackme02, etc. Figure out how to make the crackme exit with the status code 0.
Don't ruin it for yourself by reading the source code! Enjoy.