Some of the cool things about Windows Phone Mango for developers is the ability to access alarms, contacts and reminders, allowing the creation of a new category of apps. In this case, Yuv Apps have made a neat little program called 'My Assistant' and while a little bit rough around the edges is still worth a look.
The app features the ability to create two types of reminders: regular reminders e.g. notes to yourself and call reminders. The first one allows you to type out a little reminder note, time and date while the second allows you to pick a contact, date and time to call them. You then have the ability to make them active/inactive. Once set that's it--the app service now runs in the background and will pop up a reminder at the designated time. After that, you can back to the app and hit "completed" and the reminder will become "inactive".
The app could use a loading screen, a Live Tile showing how many active reminders you have and auto-suggest when typing a note, but the developers are open to new ideas/improvements and the app is free for a limited time. In that sense, we dig it and think it's worth your time if you have a Mango phone. You can pick it up here in the Marketplace.