One advantage that Mandrill has over SendGrid is the ability to see the content of emails (run a web agency so it helps a lot with troubleshooting for our clients).
https://sendgrid.com/docs/User_Guide/email_activity.html
If you're doing one-to-many campaigns, we also have a marketing email product called Marketing Campaigns that offers a more detailed content history.
(I have had many a batch email with "{{name}" or some other templating error, heh.)
Disclosure: I lead product development @ Mailgun.
I'm currently sending 150k/mo with Mailgun and log everything for 6 months. It's rarely accessed, but invaluable when it is.
Zurb Foundation has one with a web interface: http://foundation.zurb.com/emails/inliner.html
I personally prefer Jon Kemp's open source inliner, which is a good choice if you want to do that step locally: https://github.com/jonkemp/inline-css