Plan 9 from Bell Labs

A different free operating system, which was far ahead of its time. The architecture is both pragmatic and elegant (three different small but monolithic kernels for "terminals", "cpu servers" and "file servers", and most things including filesystems running in user space).

It's presumed dead by many, but there still are active developers. If GNU/Linux isn't geeky enough for you anymore, try this. If you want to make efficient use of multiple networked computers, also try this.