The first video calling app is now available for Windows Phone 7.5 Mango devices. You can find "Tango" in the Windows Phone Marketplace as a free download and it works on any Windows Phone 7.5 device. That's right, it does not require a front-facing camera and you can use the rear camera for video sharing as well. The app supports notifications, so when you get an incoming call a small pop-up will appear at the top and then you tap that to see the full incoming call dialog at which point you can press answer and it will start a voice call. The video calling needs to be turned on manually by both people from this point. So for example if the other person doesn't want you to see them right away, they can choose to not turn on the video chat until they look more presentable.
Versions of the Tango video calling app are also available on iOS, Android, and Windows PCs so you can video chat with anyone who has any of those smartphones (and the app) as well. It doesn't require sharing your username either like Skype does for example. It automatically looks at your contact lists and matches people who have already installed Tango based on their phone numbers and email addresses in your contacts list. So whomever has Tango installed will show up in your list as being call-able.