To refresh a widget in the Dashboard, open the Dashboard, select a widget, and press Command-R. There is a slick little animation, and the widget will refresh.

[crarko adds: I've seen this before, but on searching didn't find it here as a hint. Here is a previous hint with a list of other interesting modifier keys. It looks like the wiki mentioned there is no longer active, so I'll see about creating a new one. Thanks to reader Reuven Volnoboy for pointing this out.]