This block/gist mashes together a bounded force layout with collision detection to create a layout in which the gravity on each node appears to pull straight down. Basically, this is a ball drop simulation.
At large scale, this is probably not practical, as this gist creates a "magnet" node for each "ball" node. The magnet node is fixed and sits at the below the bottom of the bounds and provides personal gravity for the ball. At this scale, though, it's OK.