Can Spam
The US Federal Trade Commission has published a report (100+ page PDF) into the US Can-Spam Law.
Since it was passed spam volumes have fallen from 76% of all e-mails to 67%. There have been 21 prosecutions under the law.
The law is not a very good one, as it operates on opt-out principles, but even that has been enough to allow some prosecutions.
The FTC also makes the point that technology is providing the biggest benefit to stopping spam reaching inboxes, but that legislation is playing a role.