GIMP - GIMP is a multiplatform photo manipulation tool.
CollageIt - CollageIt is an easy-to-use and automatic collage maker, which makes photo collage automatically.
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
PhotoSpills - Easy to use PC collage software with unlimited design possibilities.
ShapeX - Collage - Free Shape-Collage Generator! – (v1.1) Save Example Images & New Examples !
Collagerator - Create amazing photo collages in few mouse clicks. Ready for download for Mac OS X and Windows.