A woman stops in a BART subway station in San Francisco to send a text from her phone. But she's not telling her friends she'll be late for happy hour. By sending a text to a location-based app, she's trying to save the Earth.

Through this "check-in" on Foursquare, that woman is making a donation to EarthJustice, an environmental law organization. This is one of the many ways that nonprofits are taking advantage of a new generation of applications that utilize physical location information to tie together the online and offline worlds. Location-based apps use GPS or other physical location information to map and keep track of users interacting with the service.

Foursquare is one of the earliest location-based apps to gain popularity. Foursquare is essentially a social network that allows users to "check-in" at a physical location by sending text messages or using a smartphone app. Using the phone's location information or GPS, the message is connected to the place. Users who check in frequently get recognition; the person who checks in the most a given location is dubbed the "mayor" of that place, for instance. The service is most popular with businesses like stores, cafes, and bars since they are semi-public areas, but nonprofits have started getting in on the action. Here's a quick run-through on how to set up your nonprofit as a "venue" on Foursquare. A Net2 Local event could become a venue, too, allowing organizers and attendees to see who was there. Check out these five ways that nonprofits are using Foursquare to engage with their supporters, target populations and the public.

In the example above, EarthJustice put up a series of billboards that asked people to check in there. The billboards presented challenging messages about the environment and allowed people to take action immediately. Each text that they received brought in a donation from an independent donor. The campaign was targeted specfically at a younger demographic because they tend to be connected with mobile devices and are usually not receptive to traditional donation calls.

Places was Facebook's foray into the location-specific field. They've now made almost everything that can be posted available to be tagged by location. Organizations can verify their addresses to establish a Place and this will allow other Facebook friends to check in there. Facebook is actively promoting the recent changes that they've made to how their location check-ins work. Currently, anyone who checks in at a Place will have that status update added to their Wall. This status update is more extensive than the single line "Facebook User likes Organization X". This means that a check-in at your nonprofit's Place gives more visibility than a Like. See this article about their "recommend this place" feature.

Gowalla is another big player on the location-based social network scene. Gowalla rewards users with virtual items that can be kept, traded or dropped off at locations. The service also has users “check in” at various physical locations, but it was originally created in order to let people share meaningful experiences in their lives with others. It is based around the idea of discovering and sharing places in the real world that users find personally exciting, interesting or fun. You can read this How To guide from Mashable to get a handle on the seven different ways that Gowalla has for organizations to connect to its users, with one of the most important steps being “claiming” your location.

There are other location-based social networks and apps, such as brightkite, that are out there, but Foursquare and Gowalla are leading the field so far. Here's a comparison of the two services to help you get an idea of the hallmarks of each. It's entirely possible that other services from internet giants like Google or Facebook will rise in prominence as they get incorporated into larger social networking platforms.

The question remains, how can your nonprofit benefit from using them. Some people have already started thinking about this question and some organizations have started using these services in ways that might inspire you. Peter Panepento rounded up a few of these ideas for a guest post on Philanthropy.com about how nonprofits can benefit from Foursquare. Zoetica co-founder Geoff Livingston dished on the same topic for Mashable. Small Act also touched upon some quick ideas for using Foursquare and Gowalla. Each article covers some ways that nonprofits can use tools such as maps and user rewards for check-ins to boost the interactivity of their online presence and audience engagement. Advice for using location-based apps for cause-focused organizations can be found everywhere from idealist.org's blog, to Beth Kanter's to Selfish Giving's.

Now that we've seen a little of what location-based apps have to offer, how do you think they can benefit nonprofits? Have you put your organization on the map, or are you still trying to find your way?