Many editors can be used to make a montage like this one. The program would have to support layers (each of the 3 pictures is on its own layer) and would have to have selection tools and a "Select" or "Selection" menu to add the blur (called "feathering" or "vignetting") around the outside of the selected person in the picture.

Photoshop, Photoshop Elements, Paint Shop Pro/Ultimate, Serif's PhotoPlus, GIMP (free) can all do this. Most of the other free programs (paint.net, photoscape, etc.) that have layers can't do the vignetting (feathering) that makes these kinds of montages really precious and cute.