Yesterday, Foursquare announced they've integrated GrubHub and Seamless directly into their mobile apps. Now when you're looking for food in Foursquare, if either service is possible for that particular establishment, you'll be presented with GrubHub or Seamless as an option. One click and you can make your order.
This seems like a great partnership for both parties. It continues to cement Foursquare as a great resource for local discovery. They want to be THE app that you pull out when you're looking for where to go or what to do. For GrubHub or Seamless, it puts their service in front of more users who might not otherwise see it.