This Web-first social-sharing service caters to serious photographers (and their fans) and offers a boatload of features for sharing images. The app looks good on smartphones, but it really shines on tablets, where large images pop off the screen. Check out other photographers' work with the Flow timeline feature, which lets you scroll through, like, and comment on recent images from other 500px users. See something gorgeous? The iPad version of the app lets you purchase and download a high-res digital version of the photo. [Free; Android and iOS]