And to add to what @Discordantsaid, if you're using Gmail or Google Apps and let's say your email is user@gmail.com, you can create aliases on the fly.
Say you're registering at BBC website, then you can write your email is user+bbc@gmail.com and every email sent to that email will arrive to your user@gmai.com email.
Of course, you will not be able to send an email from user+bbc@gmail.com, but you will receive all sent to it.