SeattleTraffic is a widget that cycles through the Washington State DOT's four traffic condition maps. It is resizable and remembers your last view and size settings on relaunch.
Note that cycling between views can be odd sometimes, since each map is a different size. This is unfortunately something decided on by the nice folks at the DOT, so there's not much I can do about it.
RequirementsMac OS X 10.4 or later
*Previously available here