I use MessageTag to let me know whether recipients have got my email or not, and while I still wonder about the privacy implications, complaints have been few and far between. A new version, I’m told, is on its way.
Another option is DidTheyReadIt, which some folk (including myself) feel is far more intrusive: For one thing, recipients won’t know you’re tracking when/whether/how long for and where they’re looking at your email.
But there is another option: Mailinfo. Mailinfo takes quite an ethical approach: We believe in full disclosure. That means that a recipient of an email, sent by a Mailinfo user, is informed of the use of Mailinfo and what does it mean. Not just that: Therefore we made it a principle that in order to enjoy the service a user has to accommodate others wish to enjoy the service as well – namely a user can not use Mailinfo and at the same time use the mailinfo Confirmation Blocker.
Sadly, Maininfo only works with Microsoft Outlook. But it is free, and the homepage indicates they’re thinking about versions for other e-mail clients.