Although I grew up in a neighborhood where nearly every house had kids, "booing" was a new phenomenon. Essentially, you create a seasonal goodie bag (think some treats, some items from the $1 bin at Target, Michael's, etc., or whatever you want...I'm sure in some places these bags get filled with VERY nice goodies) and leave it on the door of a neighbor. You include a sign that lets them know they've been "booed" and encourages them to return the favor to someone else. They put the sign on their door letting people know they've been "booed" so others know to boo someone else. I think a lot of families will do a couple "boo bags" since all the stuff - a nice word for it - usually comes in multiples.
This year my bags will be filled with googly-eyed rings, ghost straws, wind-up mummies, slingshot spiders, halloween erasers, and some sweet and salty treats.
Maybe this is old news to everyone else, but I figured I'd encourage others to pass along the (new) tradition.
You can find examples of the "booed" sheet here.
(The image above can be downloaded from the blog "Creative Outlet" which has all kinds of other cute crafty stuff)