There are many times when you come across forum posts or websites that require you to signup before you can access their content, however if you are just going to use the forum or site for one time there is no point in creating a new user there and giving away your email address to them.
Of course you can use one of the disposable temporary email services to create a temporary email address and signup with the website.
Bugmenot.com is a useful service that will allow you to find and reuse username & passwords for 1000s of sites, which you can use to login and get your job done.
The service is a user driven community and all the username & passwords are submitted by other users.
While you use the service, make sure to not change the passwords as it may inconvenience other users, who are also looking to not register to a site.