Marine Traffic
Interactive and Live, you can click on the vessel to gather extra details. Even a picture of the vessel. Easily the best free resource I have found online.
http://www.marinetraffic.com/ais/
SailVX
Seems more technical (maybe more basic) and not live. Has technical info such as wave sizes, pressure and lots of other geometric information.
http://www.sailwx.info/shiptrack/shiplocations.phtml
