Hak5, a popular YouTube technology show has recently been dedicating a bunch of episodes to RTL-SDR projects. Recently we posted about their efforts with ADS-B, and some GNU Radio tutorials. Their most recent video discusses AIS (Automatic Identification System) which is a system similar to ADS-B which ships use to transmit their location to create a modern GPS based radar system.
The Hak5 video discusses how AIS works and also shows how to use an RTL-SDR to receive it. In their tutorial they use SDR#, VBCable and ShipPlotter to receive and view AIS data on map. Note that we also have an AIS tutorial here.