I'm sure many of you know about this, but it is just too much fun not to share.
http://www.ip-relay.com/index.htm
Pretty much its a program where an operator will relay a message over the phone to the phone number of your choice. It's fun to make the operators say stuff. They get suspicious when they hear typing in the background.
Hints to use it:
all you need to do when you open the java applet is type in the phone number, they will dial and tell you when you can relay your message.
after everything you say, type GA so the operator knows you are done.
if you want to end the conversation, type "sksk."
Have fun.