One of the best quick collage creators for iOS and Android, with an easy to use and efficient interface for adjustable grid-based layouts. Compares well and similar to PS Express, with twice its maximum images in a collage, non-square (but less adjustable) image grids.
Unfortunately, PhotoCollage also has annoying banner and post-export ads (PS Express does not). Aside from that. there are no significant functional limitations to the free version, unlike competitors like PhotoGrid and LiveCollage.