Just finishing my 2nd playthough of the Fallout: New Vegas DLC (Downloadable Content), Dead Money. Probably the best DLC in terms of quality and length, and it's one of my favorites in terms of immersion and story (which is a big part of the Fallout experience to me.)
Basically, if you've ever played Fallout 3 or New Vegas, you'll now once you beat the main storyline, you basically just run around and do side quests/explore when you're playing the game. The side quests can be quite boring and exploring gets old rather fast, so all there is left is to wait for the new DLC to come out. Dead Money came out about a week ago and I finished it within the first day, naturally (it's meant to be around 6 hours or so of playtime), and I had a blast. The story is genuinely interesting and a nice fit for the game. I don't want to give anything away, but the first couple hours are a bit boring, but once you get inside the casino it gets rather interesting.
It's definitely worth getting if you have F:NV, only $10 and it's a good 6+ hours of entertainment (plus you get a few Steam achievements, and if you're like me, thats a big deal.)
F:NV, Dead Money - http://store.steampowered.com/agecheck/app/72730/