These arcade games are free as long as you have Xbox Live. I'm going to download them now and edit the post if they are any good and/or easy to get achievements in.

The links bring you to where you can choose to either sign-in to your gamertag and add the games to your console's download cue OR just copy the names of the games down and look for them on the dashboard/marketplace.

Harms Way

Doritos Crash Course