Pentalogic provides free reminder for MOSS 2007. Although it's limited to 10 instances, but still good enough for most of small sites. However, they are not going to offer a free version for SharePoint 2010.
It's a big disadvantage for SharePoint which missed such an important feature. I don't understand why Microsoft doesn't want to implement that, because it's so easy to build one.
Just for fun and exercise, I created this open source tool EF Simple Reminder. No fancy UI. All settings are in a list.
As the first version, it's quite limited. No task reminding, no "one time sending", even no "email cc". But it provides the most important and basic "reminder" functionality, and, it's free!
Please check here for screen shots.
Update (10/10/2010): Task reminding is added.