In case you are unfamiliar with the most amazing new way to physically "check in" to foursquare here is a video that will explain it all:
Mark Your Territory is a fully open-source system but because it interacts with the physical world as well as the digital one, some construction is necessary beyond just simply downloading the app to your phone.
The main project site is here: http://www.markyourterritory.org/
This project uses the brand new Android ADK (Accessory Development Kit) platform for combining Arduino input with an Android device. Thus the included source code is good to experiment with and provides a basic starting point for anyone getting into
-Using Google's new Accessory Development Kit
-Using the foursquare API
-Printing colored designs with a laser cutter
(though my code is pretty messy)
If you are interested in the code aspect of this project check out the step labeled "Hacking the Code"
*NOTE: Remember that Physical Check-ins may not be recognized by many establishments. If you want to claim an area, stake something in the ground, or pee on property you don't own, you are taking your own legal risks. I'm not responsible for any kind of mess you may get yourself into.