The Internet Explorer 7 Team posted an amazing list of must-have Add-Ins for IE7. Some I'd seen, some were new, and I'm using almost all of them now.
If you're not surfing the web with IE7 yet, you can get it here, or Automatic Updates will push it out to you in a few weeks, as described in this blog article.
I have both IE7 and the Firefox 2 beta on my laptop, and I must say I am very impressed by both. Two of the must-have add-ins for IE7, Inline Search and IESpell, drop into IE7 the two missing bits of functionality I coveted in Firefox - the search filter bar at the bottom of the screen, and the spelling checker. Although some reports are that Firefox renders some sites more quickly, one IE7 feature that I find greatly increases my web surfing productivity is the Quick Tabs view (hotkey: Ctrl-Q).
For web developers, the two must-have IE7 Add-Ins mentioned, the IE Developer Toolbar and Fiddler, are tried and true additions to my arsenal.
What are your favorite and must-have browser Add-Ins?