Some web sites collect and then sell email addresses to spammers. Be suspicious of sites offering anything FREE or requiring you to register and supply an email address to gain access. Also be wary of sites offering to send a joke or picture to "10 of your friends". My favorite way around this is to use a "disposable" email address.
Get a free email address from places such as: Hotmail, Yahoo, Netscape or others. Use this address when filling out online forms that request an email address. When that address starts filling up with spam, simply abandon it and create another one to use.