We just completed a comparison of Mandrill and SendGrid here: indiemark.com/...
Ive used Send Grid for mingletrips.com. Works great. We are going to use Amazon SES since we are hosting the next app on EC2. Will give my feedback on SES.
Thanks @loren, I have updated this comparison table accordingly.
MailChimp has a new SMTP service with a free plan now, called Mandrill (mandrill.com), and the STS product is being phased out. Let's get this comparison up-to-date! I've already added Mandrill and removed STS from the larger comparison at: transactional-emailing-providers-mailjet-sendgrid-critsend
Awesome service! Blows away the incumbents and has mobile monitoring apps! You MUST try it!
SendGrid, Mandrill and Mailjet are not the only transactional email providers with free plans. Can you open this table to other popular providers? (Disclosure, I work for Mailgun)