Is there an app that lets you code and send HTML e-mail from your Mac? 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.
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.