A great little utility called smtp4dev. It's an SMTP server that listens on your local machine, and instead of relaying e-mail, it'll capture them and store them in a queue so you can review and open them.
It's brilliant – simple and elegant and incredibly easy to use. Just configure your application (website, debugger, logging framework – whatever it is you're building) to send mail on localhost:25, fire up smtp4dev, and watch the messages pile up.
Binaries and source are at smtp4dev.codeplex.com – well worth a look if you ever write software that sends e-mail.