iOS: iOS's default photo management app is well-regarded, but fairly bare bones. If you keep several years' worth of photos in your camera roll, Photowerks is a must-have app to make sense of them all.
At first blush, Photowerks just looks like a re-skinned version of the default photos app. Open up the sidebar though, and you'll start to understand what it's capable of. With just one tap, you can organise your entire library by date, city, state, country, camera make and camera model. A $0.99 in-app purchase lets you mix and match these variables to create smart albums. For example, you could pare down your library to see only photos taken on your February trip to California with your Nikon, while excluding those from the same trip that you took with your iPhone. It might seem like overkill, but I've met people with literally thousands of photos on their iPhones, and for them it might be really helpful.
Photowerks is an awesome second homescreen app, but there are a few issues which prevent it from being a full-on camera roll replacement. It can't access your Photo Stream, so you're limited to photos stored locally on the device. It also doesn't have great sharing options once you open a photo. You can't, for example, send it to Instagram, Photoshop or even a text message recipient. Your choices are basically email, Twitter and Facebook, which is a pretty big drawback. Nonethless, if you need to find a couple of photos that you took on holidays a few years ago, Photowerks is absolutely worth a download.