In addition to free Halo 4 DLC, Xbox users also have a Gears of War: Judgment freebie. Today marks the release of Judgment's first DLC, which brings Execution back into multiplayer rotation. That part's paid content, but the "Haven" map that's included isn't -- not at the moment, anyway.
In addition to the Execution add-on, People Can Fly released a new multiplayer map boasting a more snowy setting, and that's the free portion. Note that the whole "free" thing is only for a limited time, though there was no mention of when the map would become paid. Just to be safe, I'd go ahead and download it today.
Also for a limited time? A 200% XP Boost through special Haven and Executive playlists. If you're a VIP pass holder, that's a whopping 300% boost.
If you're interested in redeeming the free map or the Executive DLC pack, you'll have to do that directly from the game's title menu, Xbox 360 dashboard, or Xbox.com. See, the dialogue box that pops up during matchmaking is currently broken, but a fix should be issued relatively soon.