Rated 5 out of 5 stars
I use this to send reminders of a monthly meeting because apparently, it's too difficult for some people to put on their own calendars or in their own phones.
I set it to send a message 1 week before, 3 days before, and the day before (3 separate emails). I set it to send the email to their work address AND (and this is awesome) to their phone! Google "send sms message from email" for instructions on doing this. Each phone carrier has their own address, but basically it's [email protected] .com or similar.
Cons: It takes a bit to set up, no GUI (but good instructions with lots of options), and if you want to turn it off for a month, you can, but then you have to re-setup each reminder.
Still, it's a great addon. If there was anything better out there, it might get 4 stars due to the cons, but this is the best option I've found, and it works.