PiSpeak: Let Raspberry Pi say its IP address

Why do this? Sometimes we face the problem that how can I get the Raspberry Pi's IP address without a screen and unable to view the IP through the router? This is really a big problem, especially for the newbies. For people who are very familiar with the Raspberry Pi or other embedded systems, they would use the serial port to see the ip address. However, there's not always a USB-TTL serial tools....