I guess it would've been too big to put it in that post, but my favourite DRM approach has gotta be for Spyro 3 YOTD on PS1. The anti-reversing tricks the developers used had hackers stumped for over two months trying to crack it. There's a great writeup on Gamasutra:

www.gamasutra.com/view/feature/131439/keeping_the_pirates_at_bay.php