Rainlendar Lite

Rainlendar Lite is a feature-rich computer calendar that is lightweight and uses minimal system resources. It stores all the events in the standard iCal format, which makes sharing them with other similar applications possible. You can associate different appearances to different events, add sound alerts to remind you of certain tasks or appointments, and a host of other features. Some features are unavailable in the Lite version, compared to the available Pro version; see a comparison of the Lite and Pro version here.