Windows only: Marxio Timer is a Swiss-Army-knife like portable multifunction timer. If you only add a single timer to your virtual toolbox, this one covers a lot of bases.
As seen in the screenshot here, you can set quite a host of options for your timer. You can set Marxio Timer to perform a preset action or set it to execute an application or keystrokes of your choice. You can specify a time or have it perform the action after a recurrent event like your computer idling or when CPU usage reaches a certain percentage. Marxio Timer has password protection and will even take a screenshot at the moment if you need to document it. One of the handier features is the ability to save your settings, a big time saver if you've created a number of elaborate timers and actions. If you have a favourite timer application, share it in the comments below. Marxio Timer is portable freeware, Windows only.