There's one point Glenn forgot to mention: GIF files support multiple "frames," which means they can be used for simple animation. I believe this feature is exclusive to GIF since, until Netscape supported it, no one had bothered with this rather obscure part of the GIF specification. PNG was designed before Netscape supported multi-frame GIFs, and the designers didn't see the point of supporting this completely unused feature. Pity.
Option-Click in Scroll Bars for Jump Scrolling
In Mac OS X in general, and thus in most native Mac OS X applications, hold down the Option key and click anywhere in a window's scroll bar to jump to that spot (rather than scrolling one screen). If you like this behavior, you can make it the default in the Appearance preference pane. For "Click in the scroll bar to:" select "Jump to here."
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