If you've never edited Wikipedia, give it a go. It's way simpler than you probably think it is, especially for the often undervalued work of citing sources or fixing grammar and/or punctuation. You're likely going to read a bunch of Wikipedia anyway. Take the extra few minutes to help fix problems when you see them.
A great run through many of the noodly little details of Unix processes, signals, alarms, select, poll, and more. Includes a go through the self-pipe trick which I hadn't thought about until now, but is a neat way to simplify. I'm really happy when someone explains something by working a solution to a problem with all the trade-offs left in. Doubly so when the explanation also includes all the hairy bits of the implementations instead of leaving them as a stumbling exercise to the reader.
