Adventure Contact - Collection of Yahoo groups intended to match activity partners.
Buddyup - Find another person with the appropriate skill doing your sport in your area. Over 20 sports including running, biking, skating, golf, tennis, and skiing.
Match Maker - Find a player, team or partner to play with by registering.
PlayerLists.com - Free service allowing any amateur player of any age to find a team in many sports. Visitors can post "Player Resumes" or "Team Ads" and be notified when a match is found.
Women's Exercise Network - Locating partners for a variety of sports, primarily for women. United States.