In the event that you've never purchased any of the Fallout 3 downloadable content, today is the perfect day to do so. Operation: Anchorage, The Pitt, Broken Steel, Point Lookout, and Mothership Zeta have all been reduced to 400 points, but only until the end of the day.
If you intend to buy all of them, though, picking up a copy of the Fallout 3 Game of the Year Edition is probably a better choice.