Source Code
The source code for Ubuntu CE is made available from the official Ubuntu source archive. There are also a few packages that are specific to Ubuntu CE (i.e. Dansguardian GUI) that are not available from the official Ubuntu source archive. The source for these packages is provided in each iso download.
Ubuntu CE is 99% Open Source. There are a few packages that are proprietary. These are limited to a few themes, artwork, and programs which are all added with the consent of the Copyright holders.