Probably not the answer you were looking for, but one of the best ways is to simply just _make_ something of your own. At the very least it's good practice, and you just might make something people find useful if you've found an 'unscratched itch'.

Even failing that, your codebase can be an educational tool for others. rosettacode.org is a pretty solid example of this--any addition to the wiki is going to save someone's bacon at some point.

But to be more specific for a moment: I see you have a functional-ish language listed (Ocaml) and you've also mentioned compilers--If you're looking for low level stuff with a hint of FP, diving into Rust might be interest you. I'd suggest looking at Redox OS--it's a monster, but plenty of the big scary parts are modularized away. Might be interesting.