I decided to do some research about this hobby and was very surprised to get 22,500 hits on the Google search engine. One site stated that there were over 20,000 participants including whole families in the UK alone. Another revelation was the large number of groups that were active "behind the Iron Curtain" during the Cold War.

Some good starting points are:

http://ancienthistory.about.com/cs/reenactment1/

Society for Creative Anachronism:

http://www.sca.org/

Authenticity-Oriented Medieval and Renaissance Reenactment/Living History Groups (long list of websites):

http://moas.atlantia.sca.org/topics/groups.htm

Groups located in the UK:

http://www.the-reenactor.co.uk/

http://m.webring.com/hub?ring=ukreenactmentweb&id=13&hub

http://www.livinghistory.co.uk/

http://www.re-enact.com/

Groups located in the US:

http://www.nares.org/nares/main.htm

http://www.collectorsnet.com/cwtimes/groups.htm

War of Independence groups:

http://www.sar.org/colors/reenact.htm

Groups located in Canada:

http://www.canadiansoldiers.com/reenact.htm

http://www.canuck.freehosting.net/reenact.htm

UK magazine for re-enactors:

http://www.skirmishmagazine.co.uk/