I found this video series (https://www.youtube.com/watch?v=F3ntGDm6hOs) very interesting. It is someone (Casey Muratori) building a game from scratch using C.

The intro videos really opened my eyes to how powerful C is. Being able to directly access memory seems crazy for someone who is just used to dealing with languages that really remove you from the hardware.

+1. Learning C is not about learning a language. It's more about how to think like a computer.

This was my first thought as well- it's tough to go wrong by learning C. For me learning C was a brutal struggle (driven by necessity), but no other language taught me more about what is going on under the hood in software. C has withstood the test of time and will ground your programming skills better than any fad language or framework.

I second this. After using PHP, Java, Ruby, Python, JavaScript and various frameworks within each, "The C Programming Language" was very refreshing. It's fundamental, and made me think differently when using standard libraries in other languages, such as how they may work, etc.