[jimpudar]

Not exactly a "lesser known" area, but computer architecture and operating systems. No matter what you are doing with computers, no matter what high level language you are using, you will eventually need to understand what is happening "on the bare metal".

[ikeyany]

Computer architecture was my undergraduate focus and sometimes I'm not too sure what's happening on the bare metal. You'll want to take VLSI, semiconductor physics, and a course on compilers to get the entire picture.

[godelmachine]

I would also suggest Digital Design.