Here are some of them:
The Digital Millenium Copyright Act(DMCA): If you make of copy of anything that is copyrighted that isnt yours, you are in violation. This means 5 years in federal prison, and a $250,000 fine for the first offense.
How does this affect you? If you forward that email joke and the author gets upset, you are guilty.
The No Electronic Theft Act(NET): If you make a copy of a song to give to a friend or relative, or if you download a video, or transfer a movie that you own to a portable device(whether or not you lend it to someone) you are in violation.
If you are traveling with electronic gadgets(laptops, PDA’s, digital cameras, etc.), Customs and/or the Border Patrol(even the TSA) have the authority and the willingness to order you to allow them to inspect your device before you are allowed back into the United States. Courts have upheld that the 4th Amendment to the Constitution does not apply to personal effects at border crossings.
And, finally, Cyberstalking and cyberbullying. While federal law doesnt forbid you from writing that angry letter to a business you are unhappy with, several state laws are broad enough to interpret an angry letter as “stalking/bullying.”
Just another reason to be polite.