Sometimes you are on a website that insists that you provide an email address to register or do business with it. Given the reasonable fear of a spam barrage, you might not want to part with your email address so readily.
One option is to have a secondary email address to use for sites you worry will hit you with spam. But there is another, instant, option.
At Mailinator.com, you can make up an email address, in the following form: [any letters or numbers you want]@mailinator.com, and supply it to the website. It can be [email protected] or [email protected] or whatever you like. When email is sent to that address, you can access it by going to www.mailinator.com and entering your made-up email address. In effect, this gives you an instant, free email service, but it is only as secure as the imaginativeness of the email address you provided. Anyone who goes to www.mailinator.com can see the email sent to you if they guess the email address you made up—but only for about two hours, at which time the site deletes the email and the account.