Halo 3's former $10 set of maps for the cost of air (but not PerriAir; that the Spaceballs still charge for). The pack includes Standoff, Rat's Nest and Foundry. While we'd love to tell you all about these levels, we're getting our first taste of them today.
With all that said, there's currently a hiccup in the system and the content – which was supposed to be free at 2AM pacific last night – is still listed for $10 on Xbox Live Marketplace. Bungie's forum has a nice thread (about to break 100 pages) on the issue and we're trying to get an answer from Major Nelson for the Microsoft angle. We'll update when the map pack actually becomes, you know ... free.
Update: It's free now, go get 'em.