"MockSMTP is a native Mac application that embeds its own SMTP server. It also features an e-mail client browser, enabling instant viewing of both raw content and HTML rendering, so you can see how your mail looks when delivered." What an excellent idea.

Mailtrap helps you test ActionMailer: "Yesterday I mocked up the simplest, dumbest, Ruby SMTP server you can imagine. It speaks just enough SMTP to allow ActionMailer to make a connection and send it a message."