Microsoft is adding a little extra incentive to the Games for Windows marketplace, with an offer of free Microsoft Points for purchases. From today through August 1, purchases from the Games for Windows marketplace will net you an e-mail code for 400 MSP (or $5 in human money).
In much tinier text, Microsoft outlines the fine print. The offer is good to the first 2,500 US residents, age 18 or over, who complete an online transaction. You can only get one code per person and it will be delivered within 4-6 weeks. If you're looking for quick points, this probably isn't the way to go. Plus, make sure you double-check that e-mail address, because the site warns that if the code bounces back as non-deliverable, it won't be re-sent.
But hey, if you're $5 short on an Xbox Live Arcade game and planning to buy a PC title anyway, it's hard to argue against the value of free points. A good place to start looking would be the Marketplace's "Bargain Games" section.