Not exactly an open-source project but id software releases their source code under the GPL license after a couple of years. So you might want to have a look at

Not exactly an open-source project but id software releases their source code under the GPL license after a couple of years. So you might want to have a look at the sources of say, Doom or Quake. Both of them were written in C. In fact everything they did up to Doom 3 (C++) is C code if I remember correctly.