Dia is roughly inspired by the commercial Windows program 'Visio', though more geared towards infoDia! free flow chart diagrams softwarel 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.
well...and its totally free download here: http://live.gnome.org/Dia