People often ask about where to report spam, sometimes even sending it directly to CAUCE (please don't, we can't do anything with it).
Here are a few places where you can send your spam. Remeber to forward the entire spam, including the full headers.
Directly to your ISP (try abuse@YourISP.com) - Also, use the 'report spam' button in your webmail client. This realy does work, both in terms of blocking the spam, and sometimes in having legal action taken.
SpamCop has a reporting service (sign-up required)
United States : email@example.com
France : SignalSpam
Spamlinks maintains a great list of country-specific spam reporting services.
Netcraft : firstname.lastname@example.org (reqistration required)
U.S. CERT : email@example.com
Anti-Phishing Working Group : firstname.lastname@example.org