The Linux kernel (Popular. Not hot nor trendy). Over the past 12 months, I've touched Redis, Node.js, Clojure, and a few others on this list, but nothing blows my mind more than learning a new system call or OS feature.

Learn the exact copy on write semantics of forking (What happens to file handles? Threads? How does the OS know a paged needs to be copied?), copy two file descriptors with splice, put something in shared memory, move messages over an IPC queue.