Intersects the Foursquare todo list of you and one of your friends.
Check out a running version here: https://todosincommon.herokuapp.com
Note that this is all very hacky, just a fun little side project for my own amusement.
In order to run your own register an app here: https://foursquare.com/developers/apps
If you are deploying on heroku:
heroku login
heroku create
heroku config:set FOURSQUARE_CLIENT_ID=<yourclientid>
heroku config:set FOURSQUARE_SECRET=<yoursecret>
git push heroku master