Firefox only (Windows/Mac/Linux): Firefox extension Less Spam, Please adds an item to your right-click context menu to automatically generate an anonymous, disposable email address so you can try out a new service without fear of spam.
The extension integrates with temporary email services YopMail, Humaility and Mailinator (you can pick your preferred provider in the extension preferences). To generate a new disposable address for a site, just right-click in any input field and select Insert a temporary mail address for this site. In addition to temporary address insertion, the extension also adds an item that will take you straight to the temporary inbox for that site.
If you're simply trying to avoid registration and don't mind sharing a login with others, Firefox extension BugMeNot has always been our go-to favourite, but in other circumstances, Less Spam, Please is a worthwhile, time-saving tool to have on-hand. Less Spam, Please is a free download, works wherever Firefox does.