If you have a site which demands that you give them an email address and then respond to an email sent to that address, then Mailinator might be perfect for you.
Mailinator allows you to make up any email address at mailinator.com. You could give them an email of firstname.lastname@example.org and mail would come to that email address for a few hours, up to 24 hours, and then the email and email address would be deleted. They have a nice FAQ here.
I think it’s worth a try for those sites which you plan on visiting just once or twice and don’t want to be on their email marketing list.