One of the nice additions to the Revit 2009 SDK is the "AddIn Manager" tool.
As background, for those of you who don't know - Revit still uses a 1990s-era REVIT.INI file, with four lines per command to register your Revit external commands, ie:
ECName3=Door Mark Update
ECDescription3=Revit Door Mark Update Utility
This, combined with the application startup time for Revit, contribute to some occasional pains in getting external commands set up (whether by developers or CAD Managers).
The Add In Manager application is itself a Revit External Command, and is accessed (after installation) on the Tools + External Tools menu:
The AddIn Manager shows you what external commands have been registered with Revit, as well as giving you the ability to Load a new command at Runtime (NO MORE EXITING OUT!) and Saving changes to your REVIT.INI file.
Chuck Han had done something like this a year or so ago - but it was mainly for outside of Revit - not something that you could load into Revit after Revit had started. So this is a welcome addition - anything to make the process go more smoothly for Revit Users and administrators!
By the way, the AddIn Manager also handles External Applications: