Nimbuzz Messenger lets you make international voice calls to any user in your contacts book without paying a cent (the other person only needs to have Nimbuzz installed as well).
You can also sync Nimbuzz Messenger with other IM tools like Facebook, Yahoo Messenger, or Gtalk, in order to create a single communication channel you can use to send messages to all your friends and acquaintances. Nimbuzz also allows you to create contact groups or chat rooms, where you can organize all sorts of events.
Among the most interesting features of Nimbuzz Messenger is the ability to save messages, even when you don't have an Internet connection. This lets you read them no matter the status of your web access.
Just like other similar apps such as Viber or Skype, Nimbuzz Messenger lets users make voice calls to mobile phones, provided that you pay a small fee. The calls, in any case, are quite cheap.
Nimbuzz Messenger is a good alternative among the plethora of nearly identical communication apps available in both Google Play and the App Store.