Where can one go to report unwanted spam emails?

There are a number of places where one can go to report unwanted spam emails in the US. A complaint can be lodged at the Federal Communications Commission (FCC). There are a number of online services such as Spamcop, which look at unrequested emails and send a complaint to the appropriate authorities. One can also send a complaint to the originating ISP who sent the email. This involves a little work as the "From" header of the email is often forged.