How to report spam
Most of the apparent information in a spam is forged by spammers since they work in the shadows. You will need special tools to analyze the spam and trace back its true origins.
If you have an email address with Yahoo!, GMail or Hotmail (and many other web mails providers), you can help them identify spam by using their "report this message as spam" functions. If you prefer to take matters into your own hands, read the following.
The first tool of choice for beginners is SpamCop.net. You will need the full header and the html body source (if it is HTML) of the email message. Hotmail and Yahoo! Mail, or Outlook, Eudora, K-mail, Thunderbird for example, offer display settings in your Options where you can see the headers in their entirety. SpamCop has a FAQ section discussing these technicalities. Simply copy and paste your spam in their reporting box and SpamCop will analyze the content and provide you with addresses where you can automatically forward your complaint. You gain brownie points when you can read "Yum! This spam is fresh!", which means it is still 2 hours young or less, and its propagation is not yet complete.
KnujOn is another organization interested in receiving your spam in order to notify government agencies, financial institutions (in case of phishing and other fraud attempts) and ISPs of their spam problem on your behalf (but anonymously). You can forward your spam to: KNUJON@COLDRAIN.NET, or, for Thunderbird users, install the KnujOn or Habu add-ons to help with the process.
I would like to point out at this stage that if you receive spam at an address account from an email forwarding service, such as sneakemail.com, spamgourmet.com or jetable.org, you are better off tweaking your spam filters provided with that service or deleting that particular email address altogether, since the email headers will contain information from their servers and you risk reporting them by accident.
If you are technically savvy enough and insist on dealing personally with the abuse divisions of the concerned ISP, you can find all the equivalent information with Network-tools.com (or try the network application Sam Spade for Windows, it contains most of the site functions and you can run it from your computer), using whois, traceroute and nslookup commands to track them down. Once the appropriate ISP hosting the originating address, the spamvertised web site and the spamvertised email address (usually the remove request) is found, use abuse.net to contact their abuse department. Since spammers often publicize only disposable web site mirrors redirecting to their real web site through links, order forms, or server/client side code, in order to hide themselves from SpamCop, human detective work is necessary to create an efficient LART. Make sure that your report is clear so the abuse departments can understand it immediately.
It can be personally rewarding to observe the account canceling process in action. Spam reporting is considered a fine art by many, and you are doing a constructive service to the internet community in general.
Note: For advanced reporting technique, visit the spamfighting FAQ.
Specific spam cases
For some specific cases, you can also forward your spam (don't forget, the entire headers and HTML body) to some governmental organizations:
In case of stock manipulation (any reference to a low priced stock is usually a "pump and dump") or other attempted fraudulent influences on the financial markets, forward it to email@example.com. This SEC page has more information. If the stock is listed in NASDAQ, inform firstname.lastname@example.org, and if its an OTC BB penny stock, also inform OTCBBFeedback@nasdaq.com. You can also send a copy to email@example.com.
For serious internet frauds (white collar crime type) go to the ICCC, which is a partnership between the FBI and the National White Collar Crime Center.
If, after all, you were defrauded by spammers, I suggest you send a complaint to fraud.org with their online form.
"Nothing in the Constitution compels us to listen to or view any unwanted communication, whatever its merit. We categorically reject the argument that a vendor has a right under the Constitution or otherwise to send unwanted material into the home of another. If this prohibition operates to impede the flow of even valid ideas, the answer is that no one has a right to press even 'good' ideas on an unwilling recipient. The asserted right of a mailer, we repeat, stops at the outer boundary of every person's domain."
- Chief Justice Berger, SCotUS