/. turned me onto the Interactive Linux Kernel map. This thing is ridiculously cool (well, for a geek like me anyways). Browse through subsystems of the Linux kernel from low level (hardware, drivers) to high level (system interfaces) and links take you directly to source code!
I don't imagine most people to geek out over this as much as someone like myself, but hey, let's face it: I eat and breathe programming and used to read Linux kernel code for fun.