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Favourite app is iTunes, but if it were an app, I'd say Exposé. The single biggest change in the way I use my computer since, well, ever.
 
Hmm, hard question. I'm going to say DVD Studio Pro. I use to do DVD authoring on a PC and thought it was OK. DVD Studio Pro is so much more efficient, easy to use and powerful that it's amazing. For non-work-related app, I'd have to go iTunes I think.
 
All Apple Pro Apps
Photoshop
Painter

Fun stuff - iStopMotion and FotoMagico by Boinx Software. AudioHijackPro.
 
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