Open Source Diagram Tools

The follwoing are some of the most appreciated opensource UML/ERD diagram tools.

DIA
http://live.gnome.org/Dia

Dia is roughly inspired by the commercial Windows program 'Visio', though more geared towards informal diagrams for casual use. It can be used to draw many different kinds of diagrams. It currently has special objects to help draw entity relationship diagrams, UML diagrams, flowcharts, network diagrams, and many other diagrams. It is also possible to add support for new shapes by writing simple XML files, using a subset of SVG to draw the shape.
It can load and save diagrams to a custom XML format (gzipped by default, to save space), can export diagrams to a number of formats, including EPS, SVG, XFIG, WMF and PNG, and can print diagrams (including ones that span multiple pages).

DB Designer

http://fabforce.net/dbdesigner4/

DBDesigner 4 is a visual database design system that integrates database design, modeling, creation and maintenance into a single, seamless environment.It combines professional features and a clear and simple user interface to offer the most efficient way to handle your databases.DBDesigner 4 compares to products like Oracle's Designer©, IBM's Rational Rose©, Computer Associates's ERwin© and theKompany's DataArchitect© but is an Open Source Project available for Microsoft Windows© 2k/XP and Linux KDE/Gnome. It is release on the GPL.

NetBeans

http://www.netbeans.org/

The NetBeans IDE is an award-winning integrated development environment available for Windows, Mac, Linux, and Solaris. The NetBeans project consists of an open-source IDE and an application platform that enable developers to rapidly create web, enterprise, desktop, and mobile applications using the Java platform, as well as JavaFX, PHP, JavaScript and Ajax, Ruby and Ruby on Rails, Groovy and Grails, and C/C++.
The NetBeans project is supported by a vibrant developer community and offers extensive documentation and training resources as well as a diverse selection of third-party plugins.

ArgoUML

http://argouml.tigris.org/

ArgoUML is the leading open source UML modeling tool and includes support for all standard UML 1.4 diagrams. It runs on any Java platform and is available in ten languages.