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.
http://cad-vs-bim.blogspot.com/2009/06/current-revit-tools-and-addons.html
Tag: free
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:
http://www.avatech.com/solutions/building-design/product-details.aspx?product=44
There is a ‘Lite’ version that anyone will be able to get for free.