You mean the inheritance tab in the entity editor? That's made with the Netron library. The netron library was open source however the author sold it to a diagramming firm:
http://www.northwoods.com/Index.htm
LLBLGen Pro uses a modified Netron library v2.2 (so it actually works) and with custom shapes.
It's still at sourceforge of course ->
http://sourceforge.net/projects/netron
It also seems to have been continued by others:
http://sourceforge.net/projects/netron-reloaded
Apparently the original author couldn't keep spending time on the project as he also had to eat, naturaly. It's unfortunate that he had to sell the stuff to a competitor, but that's life I guess.
It won't work with webforms, it uses GDI routines and other low-level stuff, so on the web you're not able to use this.