Layover is a neat photo app for iPhone and iPad (US$0.99, universal) that sounds like it might be a travel app. It's not, yet "Layover" is a pretty good name for what it does.
The app lets you blend up to 5 photos. Masks and careful editing help you create a new, compelling image. This is the kind of sophistication you expect from pro apps like Photoshop, and it's interesting to see these features trickle down to the iPhone and iPad.
The blending modes are quite sophisticated, with options like a simple layer on top of another image. Layover also offers Addition, Difference layers, Lighten, Hue, Saturation, Luminosity, Soft Light, Hard Light, Color Dodge, and Color Burn. Each effect rather dramatically affects the result.
You get some basic control of aspect ratio and you can also crop images. When you are done, share your masterwork via emai, Instagram, Twitter, Facebook, Flickr or Tumblr. You can see examples of the filters on the Layover website, as well as my gallery above.
These effects are familiar to Photoshop users, but I doubt they will mean much to regular photo enthusiasts without some explanation, and that's where Layover is weak. There is some popup help, but not nearly enough to explain the power that the app possesses. Some users could have trouble figuring out how to make the most of Layover. The app would benefit from a library of examples.
Still, if you know what you are doing, these advanced features are very welcome on iOS devices. The app developers also offer Diptic, an app for creating multi-panel images, which we were quite enthusiastic about. Layover requires iOS 5 or later.