Dia is a program for creating diagrams of all kinds. The current version can do UML class diagrams, Entity-Relationship modeling, network diagrams, and much more. The engine is very flexible and dynamically loads diagram-types from disk. It stores diagrams in an XML format, but can read and write a number of different formats.
DiaCanvas is a generalized version of the drawing canvas used by DIA. It extends some features used by DIA and adds new ones, while preserving as many of the original features as possible. This project is no longer actively maintained. It is suggested that you try DiaCanvas2 instead.
DinX (DinX is not X) is an experimental windowing system that performs clipping and drawing inside Linux kernel modules. This eliminates much context-switching between clients and the server, and makes the code small, simple and fast. It is aimed at small systems like Linux handhelds.
Drawtool is an application that reads in simple vector geometry from stdin and displays it in a gtk window. Similarly, it will write data onto stdout as generated by click-drag mouse movements. It can be used to prototype graphical ideas quickly, provide a networked whiteboard or draw pretty patterns with minimal effort.