GIMP - GIMP is a multiplatform photo manipulation tool.
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.
ShapeX - Collage - Free Shape-Collage Generator! – (v1.1) Save Example Images & New Examples !
Darktable - darktable is an open source photography workflow application and raw developer.
Photovisi - Photovisi is a free and easy to use online tool to create photo collages.