Today is Foursquare day (4/16 is four squared). The movement started with a Florida optometrist to celebrate with meetups and merchant offers in cities that include Miami, San Diego, New Orleans, Portland and London.
Interesting how a user phenomenon — yet to scale among SMB advertisers — is being initiated by the latter. Check in at participating locations to get special deals ($2 Foursquare shots in San Diego) and even a new badge. All the info and deals are here.