As an example:
The idea behind this one is to add strange places to Foursquare. I did this one right next to a bar.
How to do it (there may be a better way):
1) Get a Foursquare account. Bullshit info seems to be acceptable.
2) go to "Add Things" at the top. It says "ADD TIP...or add new venue?"
3) Click "add new venue"
4) I use google maps to find gps coordinates near the location; the coordinates are used in place of the address.
5) Fill out the rest of the info as you see fit.
6) Make sure the "push pin" is in the right spot.
7) save, tag, etc. to your liking.
Hope something funny happens.