Everytime you return to a site, you want to know what you've missed. SiteDelta provides that function for you with the click of a button.
SiteDelta installs a Delta icon at the bottom of (where else) the status bar. Visit a web site and click on the delta symbol to see all of the changes that occurred since your last visit.
SiteDelta is a simple extension and it's really a good idea, but an options dialog box is needed for a custom color for the highlights. The options dialog box would also prepare the author in case there were any more feature requests from users.
It's not quite an extension I would have to keep in my arsenal, but it's a solid extension nonetheless.