There are two ways to avoid spam when registering somewhere on the internet. First you can use Disposable Email Address Services that allow you to check an email account that is not your own and email aliases that webmailers like gmail and yahoo mail offer. I will only post free sites that offer the first service.
The procedure itself is always the same. You go to the website of the service and take a look at the requirements of the email address. For example the service www.mytrashmail.com offers only free emails that use email@example.com. After this you visit the website you want to register with and enter an email address which could be anything as long as it ends with @mytrashmail.com....Email Aliases:
This is becoming increasingly popular. I only tested it with my gmail account but friends told me that yahoo for instance is offering a similar service. You can send emails using a service called plus addressing. This is not documented yet in the help files so I explain the basic concept. Lets say your email address is firstname.lastname@example.org and you want to register at a website. You might register with an email alias that looks like email@example.com. The verification mail will be send to your inbox and you can reply to it to activate the account.