Woolly Alder Aphids produce large, white fluffy colonies on the branches of their namesake host making the branches look like they're covered in patches of white hair. A close look will reveal woolly aphid nymphs exuding strands of white waxy filaments from block-like structures on their backs.
I believe no other insect upstages Beech Bligh Aphids in entertainment value. During a visit to a Lake County, OH, park last Thursday, Jim Chatfield and I came across a cluster of these engaging aphids shaking their woolly derrieres in what appeared to be a synchronous samba. All we needed was "(Shake, Shake, Shake) Shake Your Booty" by KC and the Sunshine Band blaring in the background to complete the effect.