Based on a user request over at the creative cow, Nth Layer Selector will select every Nth layer.  
If you have a set of layers already selected it will select every Nth layer from within that selection otherwise it will select every Nth layer from the whole comp.

While on the subject of selecting layers, make sure to check out Redefinery’s excellent rd_Statesman script which lets you save the state of layers which in it’s simplest use will allow you have have selection sets.

Also check out Zorro which allows you to tag layers. You can then select layers using those tags.

Download: NthLayerSelector.jsx