Recent studies have shown that the majority of spam comes from spammers scanning web pages for email addresses. Most public newsgroups, chat boards, and mailing lists are archived on the web, so if you've ever posted a message your address could become public that way. In general, any time you share your email address, there is a chance it may be added to a bulk mailing list.
Some good online resources for avoiding spam are "Top 5 Most Effective Tips to Avoid Getting Spam Altogether" by Heinz Tschabitscher and "Building Your Personal Anti-Spam Strategy" by Michael Herrick.