File Systems
File systems are awesome! More precisely, the file systems interface in Unix-like operating systems is awesome. It provides a simple, universal abstraction over an enormous ambit of data and system objects. Access to programs and data stored on a local disk, access to programs and data stored in memory, information about the system, and allows running software distributed over a network efficiently.
For me, distributed file systems used on large clusters are especially interesting because they are at the convergence of block devices, local file systems, networking, and massive parallelism. The fact that file systems are deeply intertwined with operating systems has also helped catalyze my interest in Rust for Linux.