Pester is a freeware alarm clock utility that provides a simple way to set timed reminders, along with a few cool extras.
Of course, you can use iCal and other scheduling software to alert you to upcoming events, but sometimes these apps are overkill--like, for example, when you're just putting something in the oven, or you're trying to remember to catch a bus. Pester occupies a middle ground between iCal and more bare-bones timers (such as egg-timer widgets), letting you quickly set an event, name it, choose a countdown or a specific time for the alert, and then (if you want) pick from a variety of alert options, including displaying a message, playing a sound (a standard alert, or even opening an MP3 or image file), speaking the name of your event aloud, and bouncing the Dock icon. Pester lets you set multiple alerts (unfortunately, there's no way to edit them), and you can snooze alarms with a variety of options. One of Pester's coolest features is the ability to create a hot key that lets you set alarms from any application.
Pester is officially beta software, and the interface isn't exactly pretty, but if you're looking for a simple, reliable timer with some flexible extras, Pester is a great, no-cost choice.