How spammers harvest email addresses

How do spammers harvest email addresses ... the following is lengthy but well worth the read of the full article.
http://www.private.org.il/harvest.html

Spammers have programs which spider through web pages, looking for email addresses, e.g. email addresses contained in mailto: HTML tags [those you can click on and get a mail window opened]
Some spammers even target their mail based on web pages. I've discovered a web page of mine appeared in Yahoo as some spammer harvested email addresses from each new page appearing in Yahoo and sent me a spam regarding that web page.

A widely used technique to fight this technique is the 'poison' CGI script. The script creates a page with several bogus email addresses and a link to itself. Spammers' software visiting the page would harvest the bogus email addresses and follow up the link, entering an infinite loop polluting their lists with bogus email addresses.

From various web and paper forms.

Some sites request various details via forms, e.g. guest books & registrations forms. Spammers can get email addresses from those either because the form becomes available on the world wide web, or because the site sells / gives the emails list to others.

Some companies would sell / give email lists filled in on paper forms, e.g. organizers of conventions would make a list of participants' email addresses, and sell it when it's no longer needed.

Some spammers would actually type E-mail addresses from printed material, e.g. professional directories & conference proceedings