In the United States, the CAN-SPAM Act of 2003 is more of an opt out law than an opt in law. In other words, while it may be legal to send unsolicited commercial email in the U.S., you must include an unsubscribe link and honor any such unsubscribes within 10 days. Of course, there are many other requirements as well, such as don't use deceptive subject lines, don't use misleading header information, etc.
Here is a great summary from The Federal Trade Commission of the CAN-SPAM Act of 2003.
From Wikipedia: In the U.S. spam is legally permissible according to CAN-SPAM, provided it meets certain criteria: a "truthful" subject line, no forged information in the technical headers or sender address, and other minor requirements. If the spam fails to comply with any of these requirements it is illegal.
In many other countries, the requirement actually goes even further to prohibit the emailing of unsolicited email. In other words, you need permission before emailing to someone.