Visual Studio Extensions worth checking out...

Productivity Power Tools
Many Features. Best: Tab and Pin your Code files. Close all but pinned.

VS Commands 2010
Many Features. Best: Xaml Bindings and resource highlights

Xaml Styler
Great for formatting xaml.

Team Foundation Server Power Tools August 2011
Obviously only if you use TFS for your source control. Now with Rollback feature only previously available from the command line.

Code Maid
Many Features. Best: Cool Build progress bar with "Cancel Build" button.

Snippet Designer
For Ninja Coders who want to be super productive - craft your code snippets with this.

Portable Library Tools
One Code Library to rule them [platforms] all

Code Metrics Viewer
View Code metrics usually only available for VS Ultimate users.

[Update 03/09/2011] ...

Default Browser Switcher
Toolbar that allows 1-click switch the browser that VS uses for Web Projects.
Handy when you're developing a silverlight app with no web project attached.

Some Isolated Storage Explorers

[Update 09/09/2011]...

Visual NUnit
Nice NUnit UI test runner from inside VS. Run or Debug unit tests. Also has filters for project, namespace and fixture.

[Update 13/10/2011]

NUnit Test Application

Mobile Ready HTML5 MVC.NET

AnkhSVN - Subversion SCC Provider

ToDoManager
Nice replacement for the VS task manager -which get wiped when the .suo file is deleted. The solution is more presistent

Windows Phone Test Project

WoVS Quick Add Reference
Add quick reference to missing assembly when class referenced in code.

I've tried to keep to those extensions that are free.
... any cool ones I've missed?