Yahoo Messenger is one of the famous, here you can enter different chat rooms with different people also from different country, simply click the picture to Download. Click HERE to register.
Skype
Skype have Clearer Voice Calls and Video Calls rather than the others, simply click the picture to download skype, click HERE to register.
Tango
Tango is instant unlike the others,its too easy to use, unlike others, it don't need to register and log in... ideal for mobile use. simply click the picture to Download.