How can I wake up my Mac from sleep mode remotely?
From this post:


  Most modern computers offer a feature
  called "Wake on LAN". This is designed
  to allow a network administrator to
  turn on a computer remotely, even when
  it is turned off, by sending a "magic
  packet". This is used, for instance,
  to allow backup programs to run at
  night.
  
  Here is a Python script that does this
  (if your MAC address is01-23-45-67-89-0a:


#!/usr/bin/env python 
import socket 
s=socket.socket(socket.AF_INET, socket.SOCK_DGRAM) 
s.sendto('\xff'*6+'\x01\x23\x45\x67\x89\x0a'*16, ('192.168.1.255', 80))


Or you can use Wake550.

or use this site.