What GUI applications are there to assist in generating graphics for TeX?

I think the best option for diagrams, circuits, etc. is 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 export diagrams to a number of formats, including EPS, SVG, XFIG, WMF and PNG, as well as LaTeX formats such as PGF, PStricks and Metapost.

The list misses TikzEdt. It provides a GUI and semi automatic drawing tools. For details: TikzEdt web site. Available for linux/windows. Upon right clicking in the figure (on right) window, it offers a number of options to draw using the mouse. After drawing the tikz code will be inserted in the left window automatically. On the left panel, there are some styles/shapes etc from where the corresponding code can be inserted. The figure can be exported to number of formats like pdf, png, jpeg etc. And it is free.

Another approach is to use Inkscape. If you export to eps or pdf it puts the vector drawing in one file and the text in a tex file, see http://tug.ctan.org/info/svg-inkscape/InkscapePDFLaTeX.pdf. The integration is perhaps not as elegant as with software designed as an interface for LaTeX packages but offers the full potential of Inkscape.