When I think of collages, I think of apps that allow me to arrange individual photos into a new composition. Bazaart (free) does something else. You can take individual elements from photos and make a new photo from those parts.
Usually, this is very difficult because removing an element from a background can be difficult. Even Photoshop isn't perfect at this, but it does provide tools that make it easier.
Bazaart is the best I've seen at this task. I took a picture I had of my parrot, tapped on him, and Bazaart figured out how to grab the image, then let me put it in the foreground of an image from the desert. See the slide show below for the result. It was quick, clean and easy, and if you need to manually clean things up you can do that too.