Learn HTML & CSS, but beyond that, learn how to compose a usable, HTML email template. Custom emails are something every company will undoubetly want to use and its actually a fairly simple process to create one. Once you have the basics of HTML & CSS down, take a look at the free PDF available here. It will give you AWESOME information on how to develop them for multiple major email clients and make them look super cool in the process.
Great summary, thx for posting this! I'd like to add one more MailChimp alternative. It's a good choice if you're looking for a reliable ESP with outstanding customer support.
After having tried a number of newsletter editors of email marketing services - and found them always disappointing in terms of flexibility and design options - I stumbled upon http://freshmail.com and finally got what I need. It lets create stunning templates with a simple and neat drag&drop editor.