1: smtp4dev: Windows 7/Vista/XP/2003/2010 compatible dummy SMTP server. Sits in the system tray and does not deliver the received messages. The received messages can be quickly viewed, saved and the source/structure inspected. Useful for testing/debugging software that generates email. I have been using this for quite some time now and it is working seamlessly.

2: MockSMTP.app: 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. A nice product overall but is buggy.

3: Dumbster: The Dumbster is a very simple fake SMTP server designed for unit and system testing applications that send email messages. It responds to all standard SMTP commands but does not deliver messages to the user. The messages are stored within the Dumbster for later extraction and verification. The best option among the three for unit testing Java applications.