Viber (Windows/Mac/iOS/Android/Windows Phone/Blackberry/Nokia/Bada)
Viber has recently become the best alternative to Skype, not just through consistently good performance, but by being available on every conceivable platform. From Windows and Mac to iOS, Android and even Bada, Viber is available on a wide number of devices like desktops, laptops, tablet computers and, of course, smartphones. Since the app works based on your phone number, there is no need for registration and logging in. You get HD audio calls, video calls, the ability to send text messages and photos, and the possibility of group calls. While like Skype, all your contacts and messages sync between all of your devices, Viber has a unique feature which lets you transfer ongoing calls between devices at the touch of a button.
Jitsi (Windows/Mac/Linux)
In the world of video calling applications, Jitsi is special. Jitsi flaunts that speciality by calling itself, ‘the most complete and advanced open source communicator’. Since it is open source, you can build your own version of Jitsi for whichever platform you are using apart from the Windows, Mac, and Linux platforms that are provided. With the app you get secure video calls, chat, desktop sharing, conferencing and file transfer features. Since the app encrypts your calls using ZRTP, you can rest assured that no one is listening into your secrets. Jitsi is also one of the best applications that offers conferencing for any number of users for free. Using the instant messaging feature, you can chat with friends on any network like Facebook, Yahoo, AIM, ICQ and GoogleTalk.
Goober (Windows/Mac/Linux/iOS/Android/Web)
Goober is one of the most comprehensive and feature-rich video calling applications in the market. For starters it is available for all major platforms both on desktop and mobile. In case you are away from your device, you can even log in using major browsers like Firefox, Internet Explorer and Safari. You can use Goober for video conferencing and audio conferencing between an unlimited number of people. You can use it as a messenger for all major networks like Yahoo and GoogleTalk, you can use it as a discussion board, or you can integrate your Facebook and Twitter feeds into it. Along with the file transfer feature, you also get access to a virtual hard drive which can be used to save and share files with contacts including non-Goober users.
Google Hangout (Web/iOS/Android)
Google Hangout, part of the Google+ social network, lets you conduct video conferencing with up to nine other people either from a computer, phone or a tablet. Since it has built-in support for productivity apps like Google Drive and screen sharing, Hangout offers good team collaboration. Using the broadcast feature, you can live stream your hangouts on air using websites like YouTube.