Zorro makes selection and isolation sets easy and possible in after effects by adding  tags to layers.  Similar to the way you would tag photos in Flickr, you can tag layers in your comps and then select or isolate those layers in groups by using the tags.

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Randomly selects layers. There’s 4 options for how many layers are selected randomly:  You can randomly select most of the layers, many of the layers, not many and finally you can have it randomly select very few of the layers.

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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.

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Based on a user request here on aescripts, CreateTrimmedAdjustmentLayer will create a new Adjustment Layer and trim it to the range of the selected layers.

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Based on a request over at http://aenhancers.com Trim to Layer Above will trim the layers that are selected to be the same length as the layer above it. This should be useful for track mattes, glows, tint layers, adjustment layers, difference layers, etc.

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Based on a request over at aehancers.com this script takes the selected layers and parents each one to the layer above it.

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Adds a user definable number of frames to the in and out points of the selected layers.

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Randomly shifts the selected layers in time within user defined range.

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Super simple script that distributes layers in 3D space within set ranges. (you can set the ranges by editing the script)

Download: 3D_Layer_Distributor

Arranges Selected Layers in a Grid with a choice to arrange them by Rows first (Left to Right) or by Columns first (Top to Bottom). Layers should all be the same size.

Download: Layers2Grid

Fits selected layers (they can be different sizes and pixel aspect ratios) proportionally to fill the Comp frame by giving you the choice of either cropping or letterboxing.
Download: Fit2Frame