The Player Project (consisting of the Player robot server, and Stage 2D robot simulation tool, libstage robot simulation library). This tag is typically useful for those trying to integrate one of these tools into their robotics-related project.
The Player Project is a suite of free-software tools for performing robotics-related research and development. It consists of a networked robot server called Player, a 2D multi-robot simulation environment called Stage, and a simulation library based on Stage called libstage.
Player Project home, and the Player robot server
Previously, the Gazebo 3D Multi-Robot Simulator was part of the Player Project, but this has now become an independent project, and is tracked under the tag gazebo-simu
Closely related are the Robotics Operating System, which has its own tag ros and the Mobile Robotics ARIA robot interface which currently (as of 2019) does not have a tag on Stack Overflow.