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.
US-centric: Tired of all those unwanted catalogs jamming your mailbox? Try Catalog Choice, a free service that you can use to opt out of junk mail. Signup is free; once you’ve created a simple profile you can go in and find the company that’s sending you stuff you don’t want…