This isn't as simple as it sounds. What you really want is the last time a
"live" user accessed a document  library. Search accesses the contents of a
document library by crawling it, so every time an index crawl takes place
the usage analysis system records that as access. There is no easy way to
do this in 2007 and I don't believe its available in 2010 either.

Could you not run code that checked if the Modified field is not the
System Account and loop through every document  on every DocLib? It would be
system intensive but I think it could be done....no?

That is one approach, but would only give you the last time something in a
Library was modified. It wouldn't identify libraries  or lists that are in
constant use, but only as a source or ReadOnly information. For example,
consider a Wiki site where information input has been stabilized. None of
the pages may be being modified, but that doesn't mean users aren't reading
info off the site on a regular basis. Crawling the site to index it simply
reads the file, it doesn't modify it so the Modified field doesn't get set.

For a really good review of reporting systems available on SharePoint, take a
look at http://www.sharepointreporting.info/