Foursquare for a number of years. In general I find this to be a useful and fun application for letting my friends know where I am, although many friends express puzzlement about why I would willingly disclose my location.
Recently Foursquare started encouraging me to try out their new Swarm application. Initially I was reluctant and stuck with the app that I already knew. However,they finally disabled check-ins with Foursquare and so I was forced to install swarm.
My first impressions of Swarm are quite positive - it looks like a nice upgraded version of the Foursquare app. However, I don't see why they had to split off check-ins into a new app. It seems that mashable agree with me.