Here are all the geoprocessing wall reference posters in one place. Since 10.0, the only ones available are somewhat messy, but nevertheless useful, programmatically generated diagrams. I'm told this isn't a real Entity-Relationship Diagram (ERD) but I still think it helps give you a bird's-eye view of arcpy you can't get any other way.
Thanks Jason Scheirer for sharing these.
arcpy "XI" - ArcGIS Pro (pre-release) Uses Python 3.4
https://community.esri.com/servlet/JiveServlet/download/429775-98287/WallReference-ProApp1.0.zip
arcpy 10.3 (pre-release) Desktop 10x lives on! (Python 2.7.x)
http://www.arcgis.com/home/item.html?id=b1e16a7b04c64da591301a796fa044d0
arcpy 10.2.1 (10.2.x) SQLite support!
http://www.arcgis.com/home/item.html?id=1a07cac100e545a4b6ea0bbc245f7d3c
arcpy 10.1 Native 64-bit (10.1 SP1) arcpy.da
http://www.arcgis.com/home/item.html?id=20b26b39913c4661a2ba85674d07c2e3
arcpy 10.0 arcpy arrives - toolboxes as modules - arcpy.mapping - arcpy map algebra
https://community.esri.com/servlet/JiveServlet/download/267604-31049/WallReference.pdf
arcgisscripting 9.3 Finally, "native" arcgisscripting!
ArcGIS Desktop Help 9.3 - Geoprocessor programming model
arcgisscripting 9.2 The old PythonWin COM Dispatch interface. How far we have come.
ArcGIS Desktop Help 9.2 - Geoprocessor programming model
Related discussions
ArcPy ERD poster? (2010)
Re: Geoprocessing modules/tools/functions reference listing
