I'm not sure the innovative Color Lake app solves any real photographic problems I have, but it sure is cool to play with and try on some of my photos taken in the desert.
Color Lake (US$1.99) adds rain, snow and water with reflections to any image in your library. You can adjust the wave height, change the weather and manipulate its hue while keeping the image at the original resolution if you desire.
I have a lot of photos taken around Arizona and Utah, and loved the ability to create realistic looking bodies of water that accurately reflect the clouds and the mountains. It's not that I needed this functionality, but it works well and has made a few of my photos more interesting than they would have been otherwise. Of course the depiction is a complete fantasy.
The author claims the site can also render these scenes as a movie, but I couldn't get that to work on my iPhone 4S. I haven't seen any complaints about this problem from users, so I mention it in passing. Note that the aspect ratio of your image will change because the reflections are added to the bottom of the photo with the default settings. While most settings are obvious, the app lacks built-in help to explain everything without having to experiment.
Color Lake is a universal app. It requires iOS 5.0 or greater. All the descriptions in the world aren't as effective as seeing images created with Color Lake, so check the galleries for some examples and screenshots.