Here's a good one I found on Lifehacker a while back.
I changed a few things when I did it, like allow the text to come from any phone and send the reply to that phone, and making the SOS text a passphrase so that nobody accidentally sent it to me.
I also made one that turned my ringer up and played a song for when I lose my phone around the house. I can just text my phone 123findme and it will play a song even if I left it on vibrate mode.
Launching ConnectBot remote commands in answer to another question.