Mail

mailto:address@domain.com?subject=This%20is%20the%20subject&body=That%20is%20some%20body

This is the built-in mail application for the iPhone

Application homepage: http://www.apple.com/iphone/built-in-apps/mail.html
iTunes link:
HTML Link example: mailto://address@domain.com?subject=This%20is%20the%20subject&body=That%20is%20some%20body
Native application URL string example:
Extended example:
Tags: mail