Rainlendar Rainy - (Freeware)

Rainlendar is a free and feature-rich computer calendar, which is also really lightweight and uses minimal system resources. It is very easy to use, and stores all the events in the standard iCalendar format, which makes sharing them with other similar applications possible.

Using Rainlendar, you can associate different appearance to different events, add sound alerts to remind you of certain tasks or appointments, and even keep track on your Microsoft Outlook's appointments.