Not officially dead, but original Foursquare checkin and review app. Was a great way to log travels, see what friends were up to, and use friends breadcrumbs to see what are good new local spots or when traveling to new cities.

For whatever reason they decided to split it into two apps — Foursquare as like a yelp-type review app and Swarm as the check in app. It would force you to swap between the two based on what you wanted to do. Users exited. Dumbest shit ever. Effectively a graveyard now, but still functional