ArcGIS Diagrammer is a productivity tool for GIS professionals to create, edit and analyze geodatabase schema. Schema is presented as an editable diagram in an environment familiar to users of Microsoft Visual Studio.
Essentially ArcGIS Diagrammer is a visual editor for ESRI’s xml workspace document. These xml documents can be created by exporting the schema from an existing geodatabase to xml using ArcCatalog. ArcCatalog can also create geodatabase schema by importing an xml workspace document.
Here is a brief video demonstration of ArcGIS Diagrammer: