Just a helpful tip for those that use Office 2007 (and probably 2010):
To get the + sign to display on the header row, go into the Data menu, click the small icon at the bottom right of the Outline group, and deselect the two options shown there. Don't do anything with the Automatic Styles though.
For 2003 versions, from memory it's Data -> Grouping ->Group Options. Don't hold me to it, as it's been a while since I've used the old version. If I'm wrong it should at least give you an idea where to start.
It would be possible to use autofilters with very little changes. You would need to create a new column (to the far left probably), highlight the first row (the frozen one at the top), and go to the Data tab then select the Filter option. For those who use 2003 versions it's nowhere near as good as the later one, but it is compatible and useable.
unnownrelic: Thanks for putting this together. A very handy reference. As for putting it into a database, what sort were you thinking of? A web (XML or similar) thing, or Access?