FreeCol is a freely available version of the Colonization game created by Sid Meier in the 4x genre, it was developed in Java and is on SourceForge. So far I've found it is an amazing thief of my time, just like Civilization was before it. I enjoy it, but I know that there were issues with the content of the game as discussed in the wikipedia article.
The current version is not file compatible with the previous version meaning that saved games will not open. As it's still very much a development game there can be odd times when it hangs or performance drops drastically, partly a problem of Java which is not really ready to tune yet as the full functionality hasn't been added. Personally the odd problems haven't put me off and its really down to my self discipline to avoid spending too much time on it.