Allows you to auto-update and synchronize Visual Studio extensions (VSIX only) between multiple instances. After installing, configure your settings by going to Tools->Options->ExtensionSync. Visual Studio will persist information about your extensions every time it shuts down. It'll also monitor the settings file for changes and update extensions, if needed.
This will not synchronize extensions installed using MSIs.
Read more at http://dotnetsurfers.com/blog/2011/03/14/extensionsync-a-visual-studio-extension-to-synchronize-other-extensions/.