Although Internet Explorer 7 doesn't have as many plugins or addons as Firefox, there are addons for IE7 that will cover off most of the features that users will need covering. Over the last couple of months I've been using IE7 as my main browser, because it's the only one I'm allowed to use at work.
Martin from gHacks has put together a great list of some of the free plugins, which are all available in the Windows Marketplace. He's even located the Firefox equivalent if you want to compare the plugins side by side. My favorites from the list are IeSpell, Inline Search and Video Downloader.View
Microsoft, Internet Explorer 7, IE7, Addons, Add-ons