If you've got an iPhone - or are planning to get one when Verizon starts selling the iPhone - chances are you know about Instagram, the free photography app that lets you share your photos with your friends near and far. Like Twitter for photos, Instagram is one of the most popular apps of all time in Apple's App Store, having captured over 1 million users in less than three months.

Why is Instagram so popular? What Instagram allows you to do is upload photos from your phone, apply one of 14 (and counting) filters, then share them with your followers. You can choose to share your photos strictly within Instagram, or blast them to your accounts on Twitter, Facebook, Flickr, Posterous, Tumblr, and Foursquare. Within seconds of posting your photos, followers can "like" and/or leave comments. Notch up a dozen or so "likes" within a short amount of time, and your photo may end up on the popular page.

Instagram is simple. And it's very addictive.

Although the app's popular page regularly features photos of cozy kitties and posed bathroom mirror shots, some of the best photos that its users share provide a window onto the world. In other words, Instagram feeds the travel porn addiction in real time. A quick look at my Instagram feed as of this writing returned images of the sun setting over Eminönü in Istanbul; a train station in Moscow; a cottage in Bath, England; San Francisco's Union Square decorated for Chinese New Year; and a snow-capped Mount Fuji. Instagram makes you want to travel and it's a fantastic way to show off your own travel photos, a fact not lost on National Geographic, Instagram's first major media brand partner.

If you don't have an iPhone – but still have wanderlust – several websites, such as instagre.at, allow you to view the most popular photos of any given moment on Instagram. You can also follow the Instatips Tumblr, a collection of photography tips and Instagram photos curated by Josh Johnson, the self-appointed Instagram guru. But while these websites are handy, there's nothing like having Instagram – and the world – in your pocket.

Filed under: Gear, Photos