The Visual Studio Gallery (http://www.visualstudiogallery.com/) is website aggregating all extensions and add-ins that have been developed to work with Visual Studio 2008 and earlier versions.
This is a great resource to look for extensions going from testing tools, control libraries and toolkits, build tools (like Dotfuscator), documentation (like GhostDoc), source control, web tools (like Aggiorno), performance tools (like Dev\efor), Visual Studio templates (like SharePoint SmartTemplates for Visual Studio 2008) and much more.
And more good news? Lot of these add-ins are free!
Develop your own add-in
If you are looking into developing your own add-in you might want to check out these Chopsticks videos recorded during the VISUG event on Visual Studio Extensibility, with Jean-Marc Prieur and James K. Lau: