Tekzilla Daily Tip #30: Autostart and Auto-shutoff your Mac
While it's more expensive than a clock radio, your Mac can do a pretty good impersonation of one.
All you need to do is go into system preferences and then the energy saver icon. Select the schedule button at the bottom right. Tick the start/wake up box and select the time you want your machine to wake up. Don't forget to choose whether you want it to startup on weekends, weekdays, everyday or just one day of the week. If you want to turn itself off tick the sleep box. Again select the time and days. Finally choose whether you want it to sleep, turn off or restart.
Note: restart available only in OS X Leopard everyone else gets shutdown or sleep.