Get the Add-ons
The following are stable add-ons (virtually all freeware) that work well with Internet Explorer 7 and 8. I wrote some of these, and others are provided by others in the community.
IEToys - A small suite of powerful add-ons for simple tasks like ad removal, dictionary and encyclopedia lookups, etc.
Mouse Gestures - Mouse gestures allow you to use the right-mouse button + movement to trigger complicated actions. For instance, you could define UP + RIGHT to mean "Open in background tab". Or DOWN + RIGHT to mean "Close this page".
For Web Developers
See Web Developer add-ons.
For RSS Users
Feeds Plus offers notification of new feeds and an aggregated reading view. (IE7 only)
Feed Folder offers a "Live Bookmarks" style view of feed items from your Favorites menu. (IE7 only. IE8 has a similar feature built-in).
The old IE5 PowerTweaks offer a way to add a site to the Trusted Zone or the Restricted Zone directly from the tools menu.