Trigraphy - Photo editing app that allows you to make mosaic/abstract looking art.
CollageIt - CollageIt is an easy-to-use and automatic collage maker, which makes photo collage automatically.
Dreamscope - Make and share 'deep dream' images using dozens of supplied styles or your own.
PhotoSpills - Easy to use PC collage software with unlimited design possibilities.
Photomosh - Turn anything into a vaporwave
Collagerator - Create amazing photo collages in few mouse clicks. Ready for download for Mac OS X and Windows.