I wish there was something like BrowserStack but you send a test email and it shows you how it renders on tons of different email clients on various platforms. It wouldn't work for web-based email like Gmail but it would still be useful.
Litmus, testi@, Email on Acid, InboxMonster, SendForensics, Email Preview Services…
Most if not all also support stuff like deliverability, DMARC testing, Analytics, Accessibility, as well as web-based render testing. I think rendering engine wise, Testi@ has the largest device/platform coverage. Or at least last I checked.
