Ipe

Xfig is the drawing tool of choice for people writing papers that use the latex->dvi->ps pipeline. However, Xfig lacks a geometric sensibility; a way to draw incidences, tangencies etc precisely without having to rely on a steady hand.

Ipe is a tool first developed by Otfried Cheong that can handle such features. You can request that objects be snapped to boundaries of other objects, intersections, grid lines, etc. It is extensible (via Ipelets), and generates LaTeX for easy inclusion in files. This is a godsend especially when you have text and pictures interspersed in a drawing (Xfig can do this, but it is a little trickier and you don't really get WYSIWYG behaviour). Ipe also has plugins for computing Voronoi diagrams and Delaunay triangulations.

I've know about Ipe for a while but have been too lazy to change over; playing with it recently was encouraging, as the learning curve appears to be quite gentle.

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thanks to Sariel