If Twitter is the what, Foursquare is the where, but now there is more incentive to use the latter. They have put in more work than Twitter on the business front, allowing local businesses to give discounts and specials for heavy users.
If you aren't in the know, Foursquare allows you to "check in" to a venue and earn points, special titles (like mayor, if you are the person who has been to a particular venue the most), and even badges. It's a game, and once you get started, it's addicting.
Once it's available in more cities, and more users are hooked in, this local business idea will surely become viral and even competitive. This is just the beginning.
Doing a quick scan of #foursquare on Twitter search, I found that a place a few blocks from my office is doing a mayor special.
Do you use Foursquare? If not, move to a city that supports it. And cash in your mayor bonuses. Local businesses, jump on this link immediately, or you are missing out.