AE Scripts is on a roll. The latest is Zorro-The Layer Tagger, an embeddable panel script that "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." This one even has a couple of video tutorials; here's one on Vimeo (Motionbox was slow):
Zorro Video Tutorial from Lloyd on Vimeo.