The power widget you see here isn't really new in Android 2.2, but it's been tweaked for Froyo. The screen brightness selector now includes an auto brightness setting (using the ambient light sensor) in addition to the three static settings (low, medium and high). It's a nice little addition for those of us who prefer to let the phone determine the screen brightness.
(Thanks, Philip! Find a cool new feature in Froyo and want to tell the world about it? E-mail us here and we'll make you famous!)