Foursquare added the ability to connect Apps into their interface a few weeks ago. They’ve had an API for a long time, and developers (such as myself) were able to build apps for awhile; but their recent modification is a bit different because you can now embed your app content into the actual Foursquare app itself.
I think most of these apps appear when you check in, such as the Weather Channel’s app. Their app shows you the day’s weather upon the first check in, which is a horribly simple but brilliant idea! I was kicking myself that I didn’t think of something like that. But then I used it a few times and realized that it is actually absolutely useless.
The reason is because when you check in, you have already went somewhere (such as the mall, work etc). A weather forecast telling you that it’s going to rain is useless after you’ve left home. The only times that I check in immediately after I wake up is at hotels to maximize the travel reward points that I receive, so I suppose it is slightly useful in that case; but it is overall useless.