A group that wanders from rave to rave (I suppose it's possible that they provide DanceSafe in other places, but I've never heard of it) and will test drug purity for you. There are also kits available at dancesafe.org that will serve the same purpose.

DanceSafe justifies its actions with the premise of saving lives, adhering to a harm reduction approach to drugs, as opposed to a prohibition approach, which has been proved to fail. They claim (and rightly so) that the drugs taken most often at raves (MDMA, LSD, and marijuanna) do not kill people on their own, but that contaminants that are often used to lace the drugs kill them.

Pretty cool idea, no?