Sidebar gadgets can be a useful Heads Up Display (HUD) kind of notification system. I have used them to keep an eye on events, computer temperatures, network usage, and weather.
A great place for finding cool and innovative gadgets for Windows 7 sidebar is http://blog.orbmu2k.de/. The gadgets located there are well developed and very well polished and is my primary source for finding useful sidebar gadgets.
Scippleron also have a couple of useful calendar gadgets that give you an agenda style calendar for those of you who don't like opening up a calendar application or web app all the time. http://scippleron.com/products.php