Atevo Travel
Dozens of free searches underway on this bulletin board. Nice mix of desired destinations.

EuroTrip Find a Friend
Scroll down to the "Find Travel Friends" section. They are organized by date.

Footprint Adventures
No one else wants to go on Safari with you? This is your site!

Going Solo Travel
Singles-only club based in Calgary, Alberta. You may join their trips from anywhere. It is not a dating club.

HI-AYA San Diego
This board from American Youth Hostels is very helpful if your destination is Southern California.

HomeFree
A pay service that matches travel buddies "by religion, age, sex or affiliation." Rates begin at $45. They also have a house-exchange service.

People Brokers
Pay service based in Australia that has been in business twelve years. Remember: paying the fee does not guarantee a successful match.

Pros and Cons
A good primer on whether or not this option will work for you, from Backpack Europe on a Budget.

Roommate Locator
This free service is primarily for apartment hunters, but you might find some travelers in their ranks.

Travel Companions Wanted
A bulletin board from ETN, which disclaims responsibility for message content.

Travnet
A bulletin board service that's been online for 4 years. The most recent messages are at the bottom.

World Travel Services
This board is not really organized, so you'll have to scroll through a lot of stuff to find a good match. It does make for interesting reading, though.