If you have had the pleasure of upgrading an "inherited" site collection or content database from MOSS 2007, chances are you have had to deal with some errors for a feature or two or more.
The "SharePoint Feature Administration and Clean Up Tool" on codeplex is very helpful is alleviating the pain or discomfort caused by not know who, what, where, when, why or how all them features that failed to load on the 2007 and/or 2010 farm.
Out of the gate, there is a 2007 version to inspect and clean your 2007 SharePoint farm, web application, site collection, or site in a few clicks. By cleaning the 2007 site collections in the content database first, your Mounting of the content database for upgrade to SharePoint 2010 will go much smoother.
I hear the complaints already, but, you have a 2010 site collection that you inherited, can this tool help me clean up the "crap?" Yep! Run the 2010 version on your farm and prune away.