Based on your GPS location, Foursquare will tell you what is around you. It will point out restaurants, shops, and yes, even libraries.
Foursquare is great for libraries because it does social media marketing for you. If I were to check in at a library, it then posts on my feed to let my friends know where I am and have been. It even links to both Facebook and Twitter, posting when I check in somewhere (although this is optional so to avoid location spamming). I can offer tips and comments when I check in to a location. Libraries can use that feature to promote services, programs, or anything else that strikes your fancy. You can offer promotions to those who check in first during an event or are the current mayor. The possibilities are endless for this fun and interactive app.
I've checked in today, have you?