Another one of those things that's just too hard to get to, and buried about three levels deep in Android's settings, is manually closing an app that may be stuck or misbehaving. It happens, even with the most well designed applications, and knowing how it's done can save you the trouble of a reboot. Just tap the Menu key, then Settings, Applications, and find the app in the list you see. Tap it's entry and you'll see something similar to the above. Tap the "Force Stop" button, acknowledge the warning, and the app will close.
It's not something you want to do randomly, but times when you need it, it's nice to be able to.
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