I use email. It is my primary electronic comms channel and there's a number of reasons why it turned out to be like that. Starting with the availability of a multitude of clients. Yes, I'm not stuck to one client. I use my Outlook when appropriate, I use a Claws-Mail when appropriate, I use mutt when appropriate and I use a roundcube web interface if there's no option for any of the above left. Which gives my reachability by mail a pretty good coverage. You may have noticed this list lacking any mobil device clients. That is for a reason: There is no need for me to receive email when I'm travelling. Trust me, I will see your mail within the first hour of me arriving somewhere (in 75% of all cases). And if things are really really really urgent, just give me a call.
Anyhow and ranting aside, I didn't have a solution for electronic reminders as I do not use electronic calendars. Main reason: The aren't half as client-independent as email is. If ecalendars would have been developed back in the days of email, there'd be a protocol specification for calendar event receiving and sending (think IMAP) and everything would be legacy these days (and thus be supported). But no, calendars were never that popular as network-based applications and more of a standalone-thing. Which is why there's a zoo of protocols and sync-techniques -- and each of them is a vendor-lockin. You started w/ Microsoft Exchance Calendars? Great, you will never use their full feature set if you decide to use a Unix based client. You want to use just a Webdav-stored calendar file (webdav.html, de)? Sure but you will not be able to use every field your most beloved calendar software uses -- for mine uses different fields. So, long story short, there's no real solution. And apart from that, I'm not seeking for a fully blown calendar, just a reminder feature.
If often enough receive a mail and my initial thought is "I need to revisit this as soon as I have time." Often enough I do even already know when I will have the time. Thus the idea was to have a service to which I can forward the mail and which then bounces it back to me at that given time.
The most simplistic approach needs just an additional e-mail account (and it will work with just like any freemail service): Mails which are sent to this account are bounced back to their senders twice a day: During the time I'm on my way home (reminders for evening-todo) and midnight (rminders for tomorrow-todos). The bad news is obviously that this service will not be able to support "I want to take care of issue on weekend" for it has no concept of weekends.
Having my own domain, ad001.de, I can go a step further and dedicate a subdomain to reminder services: rminder.ad001.de takes care of that. The big advantage: I have the complete recipient realm to define the time to remind me.
First approach was using intervals -- sending a mail to email@example.com would have my infrastructure bounce it back in -- guess what -- one hour. And so on and so on. Turns out, that's nice from a theoretical point of view, but I never wanted to calculate what that means. Which is why I went on to implement weekdays, times, and dates. Thus I can now set up a reminder by using firstname.lastname@example.org; email@example.com (the actual reminder will be sent 0105 in the morning); firstname.lastname@example.org (using the next 2000, whether that's today or tomorrow).
There's one obvous question -- why sending day reminders for 0105 in the night? It's my el-cheapo workaround for the summertime switching madness. It may move to 0005 or to 0205 -- that is not a big deal, the day is still correct. Oh, and yes, I could have just used blablabla for it brings the best date/time-calculation-features. Yeah. Sure. Go do the Bannon. The inside scoop -- it works fine enough and I can live with the remaining quirks.
As I decided not to go for reinventing the square wheel I used tools availabe which turned this pretty much into a integration thingy. I use fetchmail to get incoming mails (*@rminder.ad001.de is configured to be a forward to a default address which is then polled); next step is a script which extracts the remindance date and stores the mail adequately; final step is a script which uses nbsmtp to send mails usind an arbitrary SMTP server as next hop. On receiving I already set the recipient to be the sender of the remindence-request. Everything pretty much vanilla and straight forward; lots of stuff which essentially just "needs to be done".