PyKeylogger is a free open source keylogger written in the python programming language, available under the terms of the GNU General Public License. I threw it together one day after not being able to find a simple and trustworthy (as measured by the availability of source code) keylogger for windows, and it sort of snowballed from there. It is currently available for Windows (NT/2000 and up), and Linux (using Xlib, so won't work on the console).