QuickMail DashMail might not be the prettiest Dashboard widget I've ever seen, but it certainly is one of the most functional: it allows you to send a quick email from Gmail, .Mac, Yahoo, AOL or AIM, and even has the option for entering custom server settings for those who like to blaze their own trail. Simply enter the username and password of your chosen service on the back of the widget and you're off and running. It even remembers previous recipients (sorry, no Address Book integration as of yet) and subject lines; that's what those menu ticks are beside their respective fields. QuickMail DashMail is provided free from Scue Records.
[UPDATE: Due to a company already naming an email client 'QuickMail', Scue Records has renamed their widget to DashMail.]