If you're familiar with or interested in Prolog, I would definitely recommend checking out Mercury. The language home page was just migrated and they're having broken link issues, but here's a link: http://www.mercurylang.org/ Also, you can check out the wikipedia page for a quick summary: http://en.wikipedia.org/wiki/Mercury_programming_language

The language has a lot of functionality that Prolog doesn't have and (thanks to a strong typing system) performs much better. It just needs a bigger community to support it.