So many sites nowadays require a valid email to leave a comment or get access to some features. If you only going to visit this sites once or twice or are not sure of their legitimacy and their promise that they won`t sell off your email to a third party... your best best is using a temporary disposable address.
mintemail.com is simple to use, does the job and its not blocked like some similar services out there.
This site is no longer active, since November 2011.