On the third day of Christmas, the Major sold to me...
...some cut price Dee Ell Seeeeeeeeeee!
For Fallout 3 and Fallout: New Vegas, specifically. If you haven't already gotten involved or are waiting for the complete edition of Fallout: New Vegas, there are definitely a couple of bargains to be found here. Because they vary massively in quality and value, I've tried to provide a little guidance too.
Fallout 3 DLC (400 Microsoft Points each)
Fallout: New Vegas DLC (400 Microsoft Points each)
Jon already pointed out the massive Quakecon Pack currently available on Steam, but if you were only after Fallout 3 specifically, the daily offer means you can get hold of the Game of the Year Edition for best price available. The deal on Steam means you can get hold of it for over £3 cheaper than the next best price at Gamestation. Along with the full game, you will get all of the DLC to expand the experience even further, as well as change the rather lacklustre ending the original release contained. Act quickly if you wish to get it, as the offer will expire at 6pm today. Thanks to yellams @ HUKD!