I've used the Settings plugin a lot, but it's very old and dusty. This is a nice fork of it, ported to Rails 3, and saved for future reference.

Fantastic. If you're like me, and use One Gmail Account To Rule Them All, but use Mail as your email client, you can only use the main email address to send from. This allows you to pick a from address, making your mailing lists work again, and allowing you to pick the From address on send.