Several people have asked me "What good is Foursquare?" When I first tried it I wasn't sure if it was worth my time or not, but as with many social media & cloud computing tools you play with it first and then discover how it might be used.
Foursquare is a location-based social networking service (geo-social). It uses GPS to locate nearby places, and participants "check-in" and eventually are awarded badges and mayorships. Friends can be added so your check-ins may be shared. Foursquare can be connected to a Twitter or Facebook account, so your status and comment is posted when you check-in to a venue.
As you visit places you can also add tips or suggestions. For example, I recommended the chicken chipotle sandwich at The Posh Bagel in Mountain View and the chai tea at Tandoori Oven in Los Gatos. It's also fun to be Mayor of places like Mountain View Public Library and the Hungry Hound.
Some businesses see the marketing potential and offer discounts when you visit their establishment or even when you're just nearby. I've taken advantage of a 15% discount on a restaurant bill when I showed my Foursquare check-in.
Organizations are beginning to do more creative things with foursquare - games, tours, etc. A recent blog post, http://mashable.com/2010/04/02/foursquare-brands/ talks about how some organizations are using it. Check it out!