That depends on your specification. Do you want free software? Single user or site-wide software? Do you need to access it from a wide range of locations ? Do you need support ?

Have you tried the appointment scheduler built using Excel? Surprisingly functional - http://download.cnet.com/Free-Excel-...-11657541.html (has the advantage that you can put it on a network share and therefore manage it remotely)

I assume you tried outlook? Thats a popular app used by organisations.

Google Calendar? I use this myself and find it excellent. Even has the ability to remind you by text message.