A company called Foursquare that provides GPS-based, social-networking services to mobile devices just netted its 10-millionth user — and it's only been around since 2009.

For the uninitiated, Foursquare offers users a simple bargain. Get this free GPS-connected app for your smart phone or other mobile device to announce your arrival at restaurants, bars, shops, airports, train stations, parks or any other location designated by its users, and not only can you see other Foursquare friends that might be there, but you may be entitled to specials from the vendors. The service offers tips left by other Foursquare users, rewards frequent check-ins with somewhat silly badges and, for those who check in the most over 60 days, the title of "mayor," which comes with certain privileges.


(Yes: This means other Foursquare users at the same venue can see you checking in there, although the site allows you to opt out of being publicly listed as present and–unlike Facebook–doesn't care if you use a made-up name. And yet… many Foursquare users choose to publicize their activity even more by broadcasting their check-ins to Twitter and Facebook.)

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The competition for Foursquare mayorships gets a lot of attention, but that feature isn't the real reward I get from the service. (Foursquare specials have, however, yielded me a free beer in Las Vegas and a free cupcake in Arlington, Va.) Instead, its major ongoing value to me has been the tips that have helped me find shortcuts through airports, steered me to some good menu choices in restaurants and reminded me of one-day discounts in stores. 

In other words, Foursquare functions well as a geographically defined lens to focus its users' collective experience and expertise. More specialized services have launched along the same lines, while FacebookGoogle and Yelp have paid Foursquare the ultimate complient by launching their own check-in features.

The site's success has also helped to focus people's attention on the possibilities of location-based applications.

For example, wouldn't it be great if, upon arriving at the grocery store, your shopping list popped up on your phone? Apple's next version of its iOS operating system for the iPhone will do just that by letting you set up lists or to-do items keyed to physical locations, including grocery stores or workplaces.

Other developers have been on this case for longer. Competing to-do list managers like Remember The Milk and OmniFocus have supported location-sensitive tasks for years on the iPhone. Utilities for Android and for iPhone–note that the latter requires "jailbreaking" iOS–will disable the automatic screen locks on those phones when they connect to your home's WiFi.

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A highly regarded Android application called Tasker lets you script entire actions that unfold when your phone reaches a set place (one ambitious user coded it to send a text message to his wife when his train approaches one of two stations near his home) or a particular speed (you could set your phone to read new texts aloud when its GPS says it's moving fast enough for you to be driving).

Many of these self-aware apps require an appetite for tinkering, but as people grow more aware of these possibilities these things will get easier. Apple's GPS-enhanced to-do list won't be the last example along those lines.

You may not need to trust a strange online service for these things to happen; you'll just have to trust your phone. Which, I realize, may be a stretch on some days.