Fare/Share is an iPhone (but also Android, Palm and Blackberry) app that helps New Yorkers share taxis on the go. It features one-click ride requests, fare calculations, identification by photo or apparel. No bad surprise: you can tweet your ride, select the person you ride with and every user is rated after each ride. You can even pay via Paypal!
This application seems to be really achieved, too bad I don’t live in NY, I would give it a try.
Click for more Fare / Share’s features