Hitchsters.com lets you share a cab .

hitchsters.com is an easy-to-use web-based system which conveniently connects travelers so they can share taxis (and thus split the fare) to and from an airport.  In many places taxis are the most convenient form of transportation between metropolitan centers and airports, but they are expensive, too.  The hitchsters.com matching system (patent pending) makes riding in a taxi more affordable, more environmentally friendly and more fun.

You first look for a potential co-rider by checking out the list of users who have already signed up.  If you don’t see anyone with whom you can conveniently connect, you can enter in you own information and wait to be matched.  Once a match is made, you will be notified with both a text message to your cell phone and an e-mail containing the first name and cell phone number of the other co-rider (nothing else is shared with the other co-rider).  As an additional safety feature, we preserve the phone numbers of all of the successful matches.  You then contact each other to arrange to meet to share a cab.

See Hitchsters.Com