crontab to wake osx from sleep

"cron doesn't execute while the computer is asleep, so there's no way for you to have a crontab entry to wake the computer. That said you can schedule the computer to wake just a minute or two before your cron task in Preferences >> Energy Saver >> Schedule.

It's also worth pointing out that since Tiger, Apple has moved most scheduled jobs from cron and scripts like init.rc to the launchd process. It provides more detailed (but cumbersome, I think) means of control (through several plist files). It will also automatically run tasks missed because the computer was sleeping when it awakens. Take a look at Apple's Scheduling Timed Jobs page."

You can schedule your Macintosh to wake up in the Energy Saver preference panel. See the Schedule button.