Software for drawing geometry diagrams

For geometry I've always used Geogebra, and I think it's pretty good.

Tikz is a nice LaTeX package for easily drawing diagrams. Diagrams are made by putting code directly into the TeX document, eliminating the need for extra image files. The package also is very powerful and versatile; the manual contains a very detailed description of its features.

Asymptote is my tool of choice for virtually any mathematics diagram, geometry included.