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.
Open Links from Apple Mail in the Background
If an email message in Mail includes a Web link which you'd prefer to check out after you've read all your mail, and you don't want to hunt for the link later, Command-click the link in the message to open a browser window in the background. Mail remains the foreground application, and the browser window can wait till you're ready.
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