Open Office Draw, part of the Open Office package, wasn t designed to compete with high-end graphics packages but is an easy to use, effective drawing tool that makes it simple to create flowcharts, graphs and other illustrations from a fast sketch to complex plans. It provides plenty of tools and a variety of toolbars that really streamlines the creative process.
Kivio is an easy to use diagramming and flowcharting application with tight integration to the other KOffice applications. It enables you to create network diagrams, organisation charts, flowcharts and more.
jfig is a 2D graphics and diagram editor based on the FIG file format. The user-interface of jfig is based on xfig, a popular graphics editor for the X11 window system.
Xara Xtreme for Linux is a powerful, general purpose graphics program for Unix platforms including Linux, FreeBSD and (in development) OS-X.
Formely known as Xara LX, it is based on Xara Xtreme for Windows, which is the fastest graphics program available, period. The Xara Xtreme source code was made available open-source in early 2006, and is being ported to Linux. 
Skencil is a Free Software interactive vector drawing appliction. Known to run on GNU/Linux and other UNIX-compatible systems, it is a flexible and powerful tool for illustrations, diagrams and other purposes.
A somewhat unique (for a drawing program) feature of Skencil is that it is implemented almost completely in a very high-level, interpreted language, Python. Python is powerful, object-oriented and yet easy to use.