Moodle
Moodle is a course management system designed to help educators create online courses. One of the main advantages of Moodle over other systems is a strong grounding in social constructionist pedagogy.
Moodle is Open Source software, which means you are free to download, use, modify and distribute it (under the terms of the GNU General Public License). 
Moodle runs without modification on Unix, Linux, Windows, Mac OS X, Netware and any other system that supports PHP, including most webhost providers. 
Data is stored in a single database: MySQL and PostgreSQL are best supported, but it can also be used with Oracle, Access, Interbase, ODBC and others.
Moodle is available in 34 languages
http://moodle.org/