This app has been around for a year or two and initially was bizarrely only for women. It's now for everyone and has a lot of bars in London. Usually you can only get the free drink on certain days and/or times. It's excellent I have been to many different bars! The app is sometimes badly designed and glitchy but worth persevering with. One good thing is all the bars know about it and know what to do - it's never embarrassing.
You must have Facebook as it links through that. It uses phone's location to show you a map of bars.