Loudtalks is a free application for Windows that mixes Instant Messaging and the Walkie-Talkie philosophy.
When you install LoudTalks and run it, you'll see the typical instant messaging windows but you'll realize that there's no chat window. You'll find a talk button instead.
If you want to talk to your contacts, you have to connect them as if you were using a Walkie-Talkie, it means that you have to press talk and tell your message, that will be heard by your contact on the other side.
If you want to experience a new way of instant messaging, LoudTalks can be a good choice, although at first can be a bit difficult to get used to press the buttons to speak.