Would be great if sys.databases had a column that stored the last time someone connected/used a database. I wouldn't need this to be "up-to-the-minute" or guaranteed to be accurate. In fact, just the last DAY/DATE that it was accessed would be good enough to determine whether or not a database is being actively used.
Possibly even better, the date it was last used plus a fuzzy use_count.
Aaron Bertrand did a blog post (http://sqlblog.com/blogs/aaron_bertrand/archive/2008/05/06/when-was-my-database-table-last-accessed.aspx) in how to do this using audit or index usage stats but all of these have to be setup beforehand. What we want is a way to go to a server and determine how "stale" some of their databases are.