email management by changing username

A while ago I read an article that mentioned there were online email services that allowed you to prepend a word to your email address and still get the email.

For example: if I was user@mailservice.us I could sign up here as macforums.user@mailservice.us (or whatever) and still get the email delivered.

At the time I didn't need that, but now I do. That trick has a name, and if I knew what it was called I could google for services that offer it, and how to use it.

So does anyone know what that technique is called? Or who offers it?

I vaguely recall that gmail might, but without knowing what it's called I've had no luck finding instructions or a how-to.

Thanks.