A database engine based on implicit dictionaries and red-black trees.
A long time ago, when I still had time for watching movies I had an idea.
What if I made a website where people would exchange movie ratings and would automatically correlate your preferences with other people's preferences, in other words a recommendations engine. As web hosting at the time was really expensive and I was thinking big, I decided to implement my own database which was very compact and high performance (at least that's what I think).
I also implemented captchas and my own gif encoder (relying on Apache/PHP turned out to be too slow).
It also includes some file/memory management.