Here’s an easy one — except that most of your users won’t know about it. I was just asked how someone can know about how much disk space they are using with their SharePoint sites. Well, lo and behold, if you are a Site Collection Administrator, you can (at the root of the Site Collection), go to Site Actions/Site Settings, and in the Site Collection Administration column on the far right, click the ‘Storage space allocation’ link.
On that page (_layouts/storman.aspx) there’s a summary of your disk usage and a nice interface that let’s you investigate how the space is being used.
There are quite a few useful links like this on the Site Settings page. Give your users a tour!