If anyone was looking for a free skirmish rules set for Dark Age Britain, you probably want to check out Brytenwalda.

Brytenwalda is a rules set for historical skirmishes set during the “Age of Author.”  Players take control of rival chieftains eventually forging the history of the British Isles.

It is an interesting premise, and as with many open source rules sets, it would not be at all difficult to convert it over into other historic battle scenarios.  There’s no reason this set of rules wouldn’t work for Greek or Roman battles just as well as it works for the Dark Age British.