My all-time favorite game engine is Atomic Sock Monkey's PDQ. The core version and the later PDQ Sharp are available free. You can teach or learn the core system in about 5 minutes, make characters in another 5, and start playing. It can adapt to any setting.

You want customization? It has been applied to several genres in games you can buy, some with truly brilliant setting-specific game mechanics.

It's hell and gone better than trying to bash AD&D into something that would actually fit my storyworld and gaming style.