Apple Mail doesn’t seem to let you edit the source of HTML e-mails (customised stationery workarounds aside).
Is there a Mac app that lets you send HTML e-mails?
A bit of Googling turned up HTMLMailer, but it looks like it’s no longer available.
MS Entourage sort of supported this, I don't know if its replacement (MS Outlook) still does. Either way, you'd need MS Office to use it, which probably isn't what you want.
This article also suggests a way to achieve something similar to it with Mail.app and Safari, but I haven't tried it to see how well it works.
You can code and design your Mails as HTML(+CSS) documents, open them in Safari.app and then press cmd+i to have the content automatically transferred into Mail.app
Keep in mind that not all CSS-styles are supported by all Mail-clients, a good overview on that can be found here.
Direct Mail will do what you're looking for. You can edit the HTML source code of your message by hand, or use the WYSIWYG editor to manipulate your message. Works with Apple Mail stationery, too.
When you send the message, Direct Mail will also automatically inline your CSS and apply some tweaks so it renders properly in Gmail, Hotmail, and other "HTML-challenged" email clients.
Disclaimer: I'm a developer for Direct Mail.
I had some difficulty using the CMD + I method from Safari however you can also:
There is an app called Direct Mail that has both a "lite" and "pro" versions. http://ethreesoftware.com/directmail/
I routinely use Mozilla Thunderbird for this purpose.
You can use the menu item Insert > HTML to either insert a blob of HTML or edit the entire source of your message (CMD + A, Insert > HTML)