Loudtalks Beta is a free peer-to-peer (P2P) based walkie-talkie-like push to talk application for Windows XP and Vista that allows you to talk with your online friends and buddies using an attached microphone while holding a key on your keypad, similiar to a walkie-talkie. You can even speak to multiple recipients at the same time using Loudtalks. So while working on your desktop/laptop computer, you can simply press the hotkey and speak to your colleague(s) without even opening the Loudtalks application from the system tray.

Here are the three keyboard shortcuts you need to hold while speaking to your contacts:

F7: Push-to-talk button. Hold this button while speaking to transmit your message. This shortcut works in all applications (even if Loudtalks window is hidden).
Ctrl + F7: Talk lock button. Use this combination to start sending a longer message. Use F7 to stop the transmittion.
Shift + F7: Repeat the latest message.

Download LoudTalks for Windows here.