18 U.S.C. 875 (c) – under this law it is a crime to transmit any type of communication in interstate or foreign commerce that contains a threat to injure another person. Because this law contains any communication the Internet and other electronic communication devices are covered under this law, but only if there is an actual threat to harm somebody. This law does not cover harassment or the posting of messages on bulletin boards, etc. But if somebody is convicted of this crime they can face up to five years in prison and a fine of up to $250,000.
47 U.S.C. 223 – under this law it is a crime to use a telephone or telecommunications device to annoy, harass, or threaten any person at the called number. But in order for this law to be effective the person doing the harassment has to reveal his or her name, so it would not cover cases where the cyberstalker harasses or annoys another person by posting messages on bulletin boards or encouraging others to harass the person. If convicted under this law the cyberstalker only faces a misdemeanor charge which is punishable by not more than two years in prison.
Interstate Stalking Act – this was signed into law by President Clinton in 1996 and made it a crime for any person to travel across state lines with the intent to injure or harass another person and in the course thereof places that person or their family in a reasonable fear of death or serious bodily injury. This law is covered under 18 U.S.C. 2261A. This law is usually not applied to cyberstalking cases because the person actually has to physically travel across state lines to be prosecuted.
18 U.S.C. 2425 – this law makes it a federal crime to use any means of interstate or foreign commerce (phones, Internet, etc) to knowingly communicate with a minor with the intention to solicit or entice the child into unlawful sexual activity. This law does not cover harassing phone calls to minors unless there is the intent to solicit or entice the child into illicit sexual activities.
Read more at The Legal Side of Online Stalking and Harassment.