Rainlendar is one of the best free calendar and reminder programs. Localized in many languages, this open source and easy-to-use calendar appears on your Windows desktop or in transparency in front of your other applications. It can read your appointments in Microsoft Outlook and put some colour on the concerned days,  and can even be synchronized with a server, so that you can update your data from home or from your office.

For Windows 2000, XP or Vista