Post Build 2015, Microsoft has quietly started the Office Dev Mailing List, with an introductory offer of one free year of a 365 Developer account (typically around $9/mo).
You can actually add this free year to your existing 365 Dev account
Simply sign up here using your personal MS account, log in, find the dev program sign up form, and look for the link at the top of the page that says “add to Existing Account”. Click that link, sign in to your dev account, and accept and apply the “one month trial”. You’ll soon receive a “Welcome to your trial” email confirming the one year trial period.