The Tango video calling application has surfaced on the Marketplace today, finally giving Windows Phone users the ability to engage in video conversations like (and with) their counterparts using iOS or Android. Free calls can be made between devices running those three operating systems, as well as Windows PCs, though to get the full experience you'll probably want to make sure your phone has a front-facing camera. Another requisite is that you're running Windows Phone 7.5, or Mango, which Microsoft has been rolling out to its full portfolio of WP handsets.