When Evolution or Korganizer are overkill for your needs, you may try Gremind. It’s just a simple app that runs in your system tray that provides you with nice, pop-up reminders at specified times. I could see me myself using this for smaller tasks that don’t need to go on my calendar, but that need a bit of extra prompting to do (”clean my desk today,” for example).

Project Hompage: Gremind - Home
GetDeb: Gremind - GetDeb