Even though I can't stand to use their software (or their aesthetic, or their ethos...) I'm very grateful that suckless exists. The source serves as an excellent reference if you want to learn, say, how a (non-trivial) window manager works, or how IRC is implemented in practice, or how to wrap the WebKit/GTK+ interface. The code is usually quite straightforward, and the lack of features/config makes it fast and easy to see what's going on.