Traccar is open source server for various GPS tracking devices. At the moment server supports many types of trackers, mostly cheap devices made in Asia. Versions starting from 1.0 include embedded http server and simple web application to manage devices.
Originally it was only server which was able to receive messages from some trackers (mostly cheap china trackers) and write data to database. Some people used it with their own private system, some others with opengts. As to integration with opengts, basically traccar receives gps data from trackers and writes it directly to opengts database and opengts displays those messages in web interface.
Latest version of traccar includes its own web interface (very basic, but I'm working on it ;) ), so it can work as a separate system. But also it still can be used with opengts or any other system.
PS barbudor, could you add traccar to Index for "Live GPS tracking systems"...