You are on the road with your iPhone and need to register to some site to get a password. If you don't want to use your real email address, use trashmail.
With this neat tool you are able to create a disposable email address with one button.
All mails to this address will be forwarded to your real email address for a certain number of times. When the limit is reached the email address will be automatically deleted and every following mail rejected.
Using disposable emails helps to protect you from spam mails and could be useful when subscribing to potentially "dangerous" means of communication as forums or newsletters.