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I use:

Adblock Plus
Cooliris
Fission (to put the loading status in the address bar, Safari style)
Flagfox
Forecastfox
GameFOX
Google Preview
Quartz PDF Plugin
Sun Cult
 
Kind of surprised no one mentioned Grab & Drag (http://grabanddrag.mozdev.org/index.html), it kind of makes it feel like you're doing the type of scrolling you do with the iphone (grab anywhere on the page to scroll up and down, and you can also "flick" pages (might need to adjust the friction or whatever a little to get this to work better))
 
Great addon XianPalin, thanks for that! Do you find that it confuses your orientation with a scroll wheel? I think I might have to invert mine!
 
If only there was a stumbleupon button in Safari, it is the only reason I still keep firefox around. Safari for work, Firefox for leisure..
 
Yeah Stumbleupon is my primary reason for using Firefox. I had actually forgotten that you can't use it on Safari. Real shame, great addon.
 
Great addon XianPalin, thanks for that! Do you find that it confuses your orientation with a scroll wheel? I think I might have to invert mine!

Hmmm, I've never had any problems. Just occasionally I can't select text sometimes. You can customize your toolbar up top (right click.. customize) and add a button to turn it off and on, I've found that helps sometimes. Once or twice I haven't been able to do something on a flash app because of it.
 
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