I use an app called GROUPME. It works really well and all it uses is your phone number. You can create groups for different purposes and add certain people to those groups. It's on all three formats.Skype should be able to send messages for free. Google Voice and iPhone iMessage can now do that, but windows phone still couldn't.
I am wondering, have any of you guys found a way to send messages without using a text messaging plan on a windows phone?
That is great! I am trying it out on my itouch right now. You sure you don't need a SMS plan to use it right? And that the other line receives it as your phone number?