I like the Personal Dataplan Monitor widget since I can just scroll over to the desktop it's on and have a look.

Though it is very handy I was thinking of a couple of additions that could make it even more so:

Warning and critical usage for instance where you set your plan's maximum usage and threshold for when usage is close. Something like a colour change in the text or, since this could be awkward depending on theme, a teeny indicator light.

It would be very nice as well if you could enter your monthly roll-over date as set with your contract so that the monitor resets automatically at that time (x of the month for instance).