Rminder is a free service that send voice based reminders to your mobile phone. You need enter an event in your rminder calendar. At the specified time, rminder will call your mobile phone and you will hear your voice reminder (it converts your text event to voice).
Rminder can also synchronize calendar from Backpack, basecamp, Google Calendar, Yahoo! calendar, MS Outlook and iCal. So one can hear voice reminders for any of the calendar services mentioned. The free service is limited to 8 reminders a month and is available for US and Canada only.