One reenacting period/type that seems right at home on the fringe is "pirate" reenacting. It really doesn't fit in any single category, but is something that would attract all sorts of scurvy dogs from the reenacting world.
The best site I've seen yet is Gentlemen of Fortune. It's not too detail packed, but it does link the would-be pirate to a treasure trove of resources. As a non-expert in the period and about pirates in general, Gentlemen of Fortune was an interesting site to surf, and it helped to correct some misconceptions I have about piracy and pirates during the French and Indian and Seven Years' War periods. This site has almost all the links you need to start as a reenactor or to find a group.
The Port Royal Privateers have an interesting introduction in doing a correct and accurate impression.
Although I'm not sure if actually using firearms is part of pirate reenacting, A Squared Armory does have a collection of non-firing period weapons that would top off an impression nicely.