Foursquare + LivingSocial, Gilt, and maybe Groupon. Interesting, but the execution of this will be key. If it’s simply tacked on, it could flop. If done correctly, it could be huge.
Why wouldn’t Foursquare just build a deal product themselves? Because they have 75 people. The key to all of these services is having hundreds, if not thousands, of sales people. That’s a main reason why it’s not as easy to replicate these businesses as some might have you believe.
The big challengers here will be (surprise!) Google and Facebook. Both of them have location-based services and their own (fledgling) group deals products.