iOS: We're aware that sitting all day damages your body, but reminding yourself to get out of your chair and do something isn't always easy. StandApp does the reminding for you and suggests a few stretches to try while you're up.
What's nice about StandApp is that it's not just a timer that you set to remind you to get up now and again. It also gives you ideas for things you can do around your desk when you're standing. If you have trouble remembering to stand up, or knowing what to do when you do stand up, this app is a helpful guide.
StandApp (free) [iTunes App Store]