VesselFinder is described as 'Real-time data on the positions and movements of over 100,000 vessels every day'. There are three alternatives to VesselFinder for iPad, iPhone, Online / Web-based, Windows Phone and Android. The best alternative is MarineTraffic. It's not free, so if you're looking for a free alternative, you could try My Ship Tracking.
My Ship Tracking is a FREE REALTIME AIS vessel finder tracking service. With this vessel tracker you can monitor ship positions, vessel tracking, ship tracking, vessel position, vessels traffic, port activity in realtime map.
Seasearcher provides exceptionally reliable information. And it’s the only vessel tracking tool that can offer verified port arrivals and departures, thanks to the exclusive relationship with the Lloyd’s Agency Network.