Developers building extensions can distribute them on their own (via their own web-sites or by selling them). 

Visual Studio 2010 also now includes a built-in “Extension Manager” within the IDE that makes it much easier for developers to find, download, and enable extensions online.  You can launch the “Extension Manager” by selecting the Tools->Extension Manager menu option:

This loads an “Extension Manager” dialog which accesses an “online gallery” at Microsoft, and then populates a list of available extensions that you can optionally download and enable within your copy of Visual Studio:

There are already hundreds of cool extensions populated within the online gallery.  You can browse them by category (use the tree-view on the top-left to filter them).  Clicking “download” on any of the extensions will download, install, and enable it.