A victimless crime is any action which is deemed criminal even though it is not known to cause harm to any but the participants, at most.
In some cases, more problems are caused by the illegality of these acts than by the acts themselves.
Where the acts do cause problems, it's often not clear where the blame should lie -- but current laws often blame the victim
This is not necessarily to argue that these acts should all be legal (though clearly some of them should be legal and regulated) but to suggest that perhaps passing unenforceable laws banning them is the wrong model to use, and is in a sense "passing the buck" of solving complex social problems into the hands of law enforcement.
When conducted between consenting, competent adults, the following are victimless crimes:
- homosexuality (a.k.a. Sodomy) -- jurisdictions in which this is a crime are rapidly decreasing in number
- illegal drug manufacture, usage, possession, or distribution -- some jurisdictions have rescinded such laws, and found that abuse goes down dramatically
- jaywalking, especially when the road is clear
- running a red light when all roads are obviously clear -- especially for a bicyclist or motorcyclist