Revit 2010 comes with a set of RPC content for your renders. But perhaps you want to make your own RPC content? Well, head over to http://www.archvision.com/ and register.
Then Login, and go to the menu item shown below:
Download and install the RPC Creator, and then enjoy making your own content!Have a look at this link for a getting started guide on using the program.
Please have a look at the following post for a good list of where add-ons using the API are up to at the moment.
Autodesk Seek is nearly coming of age. It references content from this site. You might prefer the interface of the Smartbim site over that of Seek at the moment.
So I got a nice email from Avatech yesterday saying that I could download the full Avatech Utilities for Revit 2010 for free!
If you are signed up with Avatech, you may have got a similar email.
Otherwise, look into it at:
There is a ‘Lite’ version that anyone will be able to get for free.