I thought I would start writing down solutions I came up while programming. I'd often come back to stuff I'd written months ago and have no idea how or why it worked. So I thought that maybe writing it down would be a good way to clear the cobwebs off my head.
These are not intended to be the best solutions to any problems. In fact they might just be the opposite...

Collision detection
Group movement / flocking
Sorting in Data Oriented Programming