The latest offer to Xbox Live Rewards members will get them free Microsoft Points just for playing Halo 4's multiplayer. If you play over 140 hours of multiplayer, you'll be given 600 MS Points. Over 70 hours will earn you 300 MS Points, and over 35 hours will get you 100 MS Points.
Spending over 3000 MS Points on Halo content in the Marketplace will get you 200 points back, and spending 1500 points will get you 100 points back. You'll also be given back 800 MS Points if you spend over 3000 points and play over 35 hours of multiplayer combined.
This offer is good until November 30, and you have to sign up for the Reward service to make it count. I won't be spending points on the Marketplace, but I'm all for the multiplayer side of this. I mean, I'm going to be playing countless hours of the multiplayer as it is, so it's cool that I'm going to be getting free money out of it.