I really like this one: http://www.laserlyte.com/products/cm-15
as it shows a pattern that gives you an idea of your spread. Also, you keep your eyes on the target and don't need to see the bead if it is dark.
However, use a white light to IDENTIFY your target as you don't want to blast a shotgun pattern in to a friendly target!
For a shotgun tri-rail see: http://www.laserlyte.com/products/adp-trir-140