GPL: Copyleft licenses - ensure that the software or derivates always stay Free too. (Linux)

BSD-Style: License that basically allows you to do everything with the software, even making it closed source, although usually some restrictions apply. (Apache and PHP)

GPL and proprietary: Dual-licensed because GPL might be "too" free. (MySQL, eZ publish)

FLOSS Exception: Exception to the GPL so that Free Software can use MySQL's client libraries. (MySQL client library)