I know the Open 7400 Contest is long past, but I wanted to call out a new 7400-based custom CPU I'm building. It's a 4 bit CPU called "Nibbler", and more info is on my web site here: http://www.bigmessowires.com/category/nibbler/
My goals are to keep the chip count to an absolute minimum, while still being able to run some interesting programs like Simon or Mastermind or Hammurabi. Everything is common 7400-series parts except the RAM and ROM, and the plans call for just 16 chips, which is very lightweight compared to past custom CPUs I've built.
I've posted some schematics and design notes to my site, and I'd be happy to have feedback. Thanks!