Yahoo's had this feature for years - You choose a separate prefix (not your actual email) and can configure '-suffix' addresses to create 'firstname.lastname@example.org' temporary addresses.
These are managed from your main email account and you can configure as many as you want, I have more than 100 of them.
You can even configure to send from the address if you like and it shows up in your From dropdown when you create a new message.
Having a prefix separate from your actual email address provides some security.
Also, having to configure them vs creating them on the fly means that if you get a spam that someone truly gave/sold your email address away.
I do wish it were easier to create addresses, like a toolbar widget or a task tray app to quickly create one in when I'm on a website that needs an email.
Better would be no prefix at all and a simple app to generate random addresses and copy to clipboard with one click... Or right-click an input field and have an option to drop an email address.