This is perhaps slightly off-topic, so apologies if it's over that particular line, but this round up of releases of game source code has been making the rounds lately: http://www.codersnotes.com/notes/the-man-behind-the-curtain
I knew about quite a few of these, in particular the id Software ones since they're famous for releasing their code, and I've read a bunch of the code reviews linked along with those (which are fantastic). Others, like Star Control 2 and some of the others I hadn't seen.
Since many of the games are old enough that they were programmed in a very low-level way like we're doing with Handmade, I figured some of them might be interesting/educational. :)