Zimride
As you can see on the image, ZIMRIDE is integrated with Facebook as an application. This is great if you are on Facebook, but do not fret if not for they are also a stand-alone website. For ease of use, you couldn't pick a better site. Their design yields a clean interface that really helps when trying to figure out a carpool situation. This site is especially good for students which could help if you are worried about who your passengers might be!