Disposable Email, fight against spam
It is used to kill spam
In reality, by using a disposable email in most of the subscribing forms, you prevent yourself from appearing on spam-lists, you fight against spam, and wild spammers
In practice
You talk with a girl (or boy) on a chat, she (he) asks you for your email address, but you don't feel it very well. Let's have a visit onto Disposable email, and give her/him the generated address. You'll get her mail, but once the delay is over, you won't be annoyed anymore
Of course, you can use it in forums, in newsgroups etc.. There are so much ways to use it that it would be boring to list them all
I use this one myself, its free and reliable and works great.
http://jetable.org/