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
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.
Many Features. Best: Cool Build progress bar with "Cancel Build" button.
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
Nice NUnit UI test runner from inside VS. Run or Debug unit tests. Also has filters for project, namespace and fixture.
NUnit Test Application
Mobile Ready HTML5 MVC.NET
AnkhSVN - Subversion SCC Provider
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?
46 minutes ago