When I do web development on my local box (running Windows 7) I never have an easy time testing out e-mail functionality, since Windows doesn't come with an SMTP server.
smtp4dev sits in your system tray and listens on port 25 for e-mail messages. It doesn't send them -- it lets you see all the message details in a neat and simple interface.
It's free, it's just what I need, and it's now a standard tool for my dev work.