Remind!

Remind! is a versatile intuitive tray-based application. It allows to create and schedule reminders than can pop up messages with optional sound effects; start applications or open documents; and shut down, reboot, log off, or power down your system. A reminder can also check for new email, dial phone numbers, synchronize system time, send messages, and open a specified Web address. In addition, you can display a stay-on-top Clock and Calendar.