From today through July 18, you'll be able to get the RDR Undead Nightmare Collection (containing three separate DLC packs) for 1,000 Microsoft Points, down from 1,600. That deal is part of a "Zombies!" promotion also featuring Dead Rising 2: Case West (now 400 Microsoft Points) and Stubbs the Zombie (800 MSP).
From July 19 through July 25, it's all GTA, with GTAIV add-ons Ballad of Gay Tony for 800 MSP, The Lost and Damned for 400, and GTA San Andreas for 800 points. See a few more items, including new Avatar items and Dance Central DLC, on Major Nelson's site.