with Reme.IO The idea is simple: you decide on your email reminder text such as 'Don't forget the dentist at 3pm' and give it your email address. You then get a reminder sent to you at the time that you have requested. Completely free - that's all that there is to it.
I have a couple of reservations - isn't that why my iPhone is for? I just tell Siri to set a reminder for me and it does it straight away. Of course, if you don't have a smartphone, ignore that concern. You can also add other people's addresses - and I can see the point of that, but it's far too easy to simply use the service to spam or otherwise irritate people - unless they send an email first to check that you're happy to receive a reminder. So, not something that I'll be using - but I'm putting it out there in case other people find it helpful.