Phonevite is a nice service if you need to send a message to a bunch of people at one. From their website, it allows you to enter up to 25 phone numbers (US numbers only) who will get the message. They then call your phone for you to record the message, so you don't need a microphone on your computer to use their service. You can then set the time at which the message will be delivered.

This is a pretty nice service for sending out reminders to groups of people.