Papyrus is a component of the Model Development Tools (MDT) subproject aiming at providing an integrated, user-consumable environment for editing any kind of EMF model and particularly supporting UML and related modeling languages such as SysML and MARTE. Papyrus provides diagram editors for EMF-based modeling languages amongst them UML 2 and SysML and the glue required for integrating these editors (GMF-based or not) with other MBD and MDSD tools. It also offers a very advanced support of UML profiles that enables users to define editors for DSLs based on the UML 2 standard and its extension mechanisms. The main feature of Papyrus regarding this latter point is a set of very powerful customization mechanisms which can be leveraged to create user-defined Papyrus perspectives and give it the same look and feel as a native DSL editor.
For more details on Papyrus, see the home page or the Wiki.