A lot of people have found ROOST through posts, pins, and other nice mentions about my easy technique for hanging a gallery wall. When I put up my hallway gallery last fall, I totally winged it with the spacing and layout.
This great diagram shows different layout and spacing options for galleries. It's actually the visual from a kit you can buy to make the exact galleries shown. I refer to the diagram to get general layout ideas, then tweak it a little to make it unique. I like the look of a slightly haphazard, collected gallery rather than something symmetrical or perfectly spaced (like the pretty galleries on my "Art" Pinterest board).