When I was looking for manufacturers of model weapons, the same makers were typically churning out replicas that were capable of firing a large plastic BB - known as Airsoft. The idea of exact models shooting out some sort of projectile isn't new. I remember my older brothers having the toy Johnny Hawk pistols and rifles that could actually shoot. As the younger brother, I was a handy target! OK, times have changed and I'm relatively unscarred by my upbringing.
Airsoft can be best described as an alternative to paintball, without the mess. Since there is no mess, then one can could go out in the field with a nice replica uniform without the worry of messing it up. This is where Airsoft satisfies a segment of the reenacting population who are into the actual moving tactics of their chosen period. The popular time periods seem to be WW2 and Vietnam. Also since these replica weapons are rated as non-firearms, fully automatic weapons are within a reasonable price and without the sticky laws and regulations you have to deal with when you possess the real deal. Here is a great discussion on reacting vs paintball.
One of the first reenactor Airsoft sites I surfed was the Airsoft Reenactors Group in CA. This is a Vietnam group that seems to do other more modern periods on occasion. The site itself is a great starting point to find out more about Airsoft reenacting, it includes impressions and event/action rules. The Gallerys are great also, and well worth some time to crawl around for a look-see.
Another site for the WW2 buff is the 2nd Rangers WW2 Airsoft Combat Team. Like the ARG site, this one is pack full with information about Airsoft and picture gallerys. The Links page is a great resource here, and it everything you'd need to hook up with a group. Unknowingly, I think a picture on this site does underscore a downside to the Airsoft hobby -- it does entail a little bit of pain. One guy looks to have a small welt on the side of his face from getting hit with an Airsoft BB. All the sites I've seen make it mandatory to wear goggles with a ballistic rating.