[Barrin92]

Implementing things that I thought only 'smart people' can do and seeing that there's really nothing mystical about it, and if you're willing to read and hack around long enough you can understand it.

Few notable things were the first time writing a ray tracer and seeing it work, writing a small emulator, writing a lisp etc. A lot of low level stuff. Learning what executables are made of on different operating systems, and so on. When you're going from not really understanding half of the stuff you use to building some of it from the bottom up, that was very valuable to me.