How spammers obtain e-mail addresses

I want to know how do these spammers obtain our e-mail addresses?

Spammers use software programs, often referred to as "spidering" programs, to search for e-mail addresses on these public forums.
Similarly, spammers have software that scans Web sites for e-mail addresses. If you have a personal web page or administrate a web site, any e-mail addresses on those pages may be located and added to e-mail lists.
You reply to spam or unwanted e-mails, asking to be removed by clicking a fake "reply to unsubscribe" link. Often when you do this, you are confirming that your e-mail address is valid and is being monitored.
This is something a spammer wants in an e-mail address.
This can result in even MORE spam.

Here are some of the Web directories that spammer may use you can use to look for an e-mail address and send fake mails:

http://people.yahoo.com/
http://www.switchboard.com/
http://www.whowhere.lycos.com/

If you have signed up for "free" or pay services on the Internet without checking the service's privacy policy and terms of use and your e-mail address was added to a mailing list.

Filling out Web site forms, surveys, etc., also can get your e-mail address added to a list.

You've intentionally signed up for a mailing list without checking their policies. Hopefully this will result in receiving only the e-mails you want and not spam. Someone else has signed you up to a mailing list and/or provided your e-mail address to a company because you didn't let them know that you want to keep your e-mail address private.

Spammers can also obtain your e-mail address without your help, simply by guessing it. Surprisingly, this is a common process, and can be quite effective. Here's how they do it: A spammer may start with a list of valid e-mail addresses, then extract the user name portion of the e-mail address, (everything before the @ sign) and try to use it with different providers, e.g., test_account@example.com, test_account@hotmail.com, test_account@yahoo.com, etc. This is a hit-or-miss approach to growing their existing mailing list that is often very successful.

Spammers also use software programs to generate random user names from common names and words in the dictionary. Again, once they generate their list of user names, they just add a popular domain name to the end.