I like looking at much better code bases than my own when I am considering making something. It's weird, but it really motivates me.

Here are a handful of ones that I really like (all related to gaming):

DOOM:

http://fabiensanglard.net/doom3/index.php - A great overview of it, mentioning the third game and its variants. Their whole github is pretty great. Just worth noting that many people consider Carmack's code to be some of the best in the industry. Really worth a read.

https://github.com/fabiensanglard/Doom3-for-MacOSX-

https://github.com/id-Software/DOOM

https://github.com/id-Software/DOOM-3

Monogame:

The Open Source implementation of Microsoft's XNA game dev framework. Really well loved C# code. And they keep it very clean.

https://github.com/MonoGame/MonoGame

Battle for Wesnoth:

Famous and well loved open source 4x game

https://github.com/wesnoth/wesnoth

Rogue-Likes:

There are a bunch of rogue-likes that are open source. It's a lovely look at the evolution of the genre and old school limitations. The genre as a whole is very open source friendly.

http://rephial.org/ - Angband

https://github.com/crawl/crawl - Dungeon Crawl Stone Soup

Lastly, here is a list of open source games. I would argue a huge chunk are not that nice. But there are gems in here. https://github.com/leereilly/games