[brenns10]

Character devices are a fertile ground for cool projects in my opinion. They're not very hard to make (typically just a kernel module, no booting a custom kernel), most unix tools interact with them naturally because they're just files, and they can do many interesting things within the kernel. One of my recent projects (after system calls) was to create a "chat server" in the kernel with a character device. Good references are Robert Love's Linux Kernel Development, 3rd edition, and the Linux Kernel Module Development Guide.