The best hands down is OSQA the english CNPROG fork, written in python. See http://osqa.net and, for an example of the site actually running: http://meta.osqa.net.

The developers for OSQA are currently building a tool to import a StackExchange dump. Two thumbs up for simple migration!

There is also a promising one in ruby called Shapado.

For a comprehensive list, evaluating project health and pointing at demos, check out this StackExchange answer: http://meta.stackoverflow.com/questions/2267/stack-overflow-...