Adventure Contact
Collection of Yahoo groups intended to match activity partners.
http://www.geocities.com/adventurecontact2/

Match Maker
Find a player, team or partner to play with by registering.
http://www.sportisgood.com/sp/Match

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.
http://www.playerlists.com/

Women's Exercise Network
Locating partners for a variety of sports, primarily for women. United States.
http://www.womensexercisenetwork.com/

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.
http://www.buddyup.com/