Honestly, you could probably even keep it closed-source, so long as people could design their own map overlays. I'm not sure how foursquare works for this - I actually tried to work with the API so my computer could send my girlfriend a text whenever she checked into a shoe store - but it seemed impossible.
If you were trying to discourage your girlfriend from purchasing shoes, this method would probably only discourage your girlfriend from checking in to shoe stores.
Granted; it was meant to be more of a humorous surprise for her. It also turned out to be much more of a challenge than I expected - there was no way to get a list of checkins for a friend, so I ended up setting up another account, friending her, and setting it to e-mail me whenever a friend checked in.
The plan was to use Lamson to parse these e-mails, but apparently it depended on a library that - in a minor version upgrade - totally fucked everything that depended on it by arbitrarily renaming several functions. Unless you specifically installed fucktits version 2.2 - not 2.1, and god help you if you installed 2.3 - it wouldn't work.
After three days, and help from a friend, I just tossed it all in the garbage and resigned myself to living under a pile of her shoes.
