Maxima. Computer Algebra System. I contribute mainly to the graphical interfaces, Xmaxima and Dynamical Systems packages.
Moodle. I have done the Portuguese translation.
Wikiup. "Wiki" system with CVS version control system (used to create this site).
psimage. Drawing system based in PostScript.
Parsewiki. A simple document preparation system, based on a text mark-up syntax.
xhtmltools. Document preparation system based on XHTML and XSLT.