Today American Express and Federated Media announced a free iPhone app, Social Currency, built on the Foursquare platform. The app lets users track things they want to buy, upload photos of purchases and comment on what their friends do, pushing the updates to Twitter and Foursquare.

The app was launched to complement Currency, a website that features financial advice for young people. The app adds a social and gaming component to the site.

"Social Currency is the app that lets you shop with all your friends, whether they're down the street or across the country. Tell them what you're buying, where you're finding it, how much you're spending, and what you want. Even better-find out what your friends are buying, too," according to a press release.

The Currency site features a Social Currency leaderboard for users with the most check-ins. Social Currency users can get "offers badges like "Thrifty Spender" and "Super Shutterbug," which the site says will soon show up on users' Foursquare badges list (UPDATE: The site originally said badges would soon show up in Foursquare; it has been changed to say they will only be viewable in Social Currency. The badges look very similar to Foursquare badges).

This new app represents another A-list brand bagged by Foursquare. American Express and its partners (disclaimer: Federated Media sells ads on ReadWriteWeb) chose the Foursquare platform over Twitter or Facebook, even though both have more users and the latter offers location check-ins.

The companies were clearly interested in Foursquare's game mechanics in addition to its social network. Users "play" Social Currency with their friends and every action results in points and "unlocking" badges.

There are arguably better apps for shopping check-ins, tracking your purchases and making shopping social. But the Social Currency app is well-made, looks just like Foursquare and includes some neat features, like adding a photo of an item you "want" to a "birthday" or "Christmas" list, and being able to comment on your friends' actions. By collaborating with Foursquare, American Express took a boring effort to "extend personal finance education" and made it social and fresh.

What do you think? Would you use this app?