Extendable, adaptable rewrite of django.contrib.admin
XDM: eXtendable Download Manager. Plugin based media collection manager.
Guacamole is an extendable and efficient rendering system for visualizing different data types.
Surikat Kung-Fu - Multisite extendable version - Framework for web developers
CarbonMU is a general-purpose, extendable MUD/MUSH server written in Ruby. Not ready for public use yet.