Sometimes you need to use your real name, and sometimes you need to use a different personality when sending your emails.
With many other email clients, it's quite easy to set up multiple sending aliases under your account, however with the Mail.app which comes with OS X, it's not possible to setup multiple personalities within the one account. If you have more than one account, you can set one personality for each account, but this gets to be a bit of a hassle if you need to maintain more than a couple of personalities.
Mail.app is a very capable email client, and even though there's no way to set up multiple personalities in it's preferences or account settings, it does have the ability to use multiple personalities.
Mailiases is a small application which leverages this hidden feature to allow you to specify your personalities, quickly and easily. Mail.app picks up the changes and allows you to select from the different "senders" when you are composing your emails.
Here's a screenshot of Mailiases in action:
