The most obvious is C, but I'm pretty sure you want something aside from that so...
A lot of people I know use Python as a hobby. I think it's OO, but I keep hearing good things about it. It seems like every new coder who I meet who learned how to code recently codes in python. Seems to be enjoying some popularity right now.
JavaScript is okay, although it really, really makes me angry with some of the features it lacks (File IO for example). But it's pretty straight forward and simple.
You could learn ASM for a classic system like the NES, Atari, or C64 or something, but that probably wouldn't count as a hobby with how much work it would be.
There's the various Linux shell scripting languages. They can do some pretty cool stuff.
If you wanted to go old school, you could learn pascal.