Here are a couple banner ads:
Feel free to use them to promote CS wherever you see fit. Also, please let us know where you're linking to CS from. We're always curious!

Banner 1:

Banner 2:

Logo - 150x33:

Logo - 200x45:

Logo - 300x67:

Logo - 500x111:

Logo - EPS:
CLICK HERE for a CouchSurfing logo EPS file (Encapsulated Post Script)