Mac: Sitting all day is incredibly bad for you, but just knowing that little fact doesn't mean we all remember to stand up and move around enough. Stand is a simple app that will remind you to get up out of you chair.
There are tons of ways to create hourly reminders to remind you to get up, but Stand does it elegantly enough that it's nice having around. Once an hour, you can get a notification to stand up. That notification doesn't go away until you click OK. That's it. It's simple, doesn't require any set up (though you can change when the hourly alerts happen), and does exactly what it says it will.