Software Applications for VOIP calls

I have only ever used Skype for making VOIP calls, but Im sure there are other software applications too. I dont know of any so please suggest what other VOIP applications there are and how good they are and there pros and cons.

Skype is pretty good but recently I started using Viber and I find it more convenient as compared to skype although nothing beats skype voice quality. Viber is like whatsapp but it also has voice so every user doesnt have a seperate ID its like whatsapp where your phone number is your id and Viber can be used to send text aswell. The best thing about Viber is that its free

So basically Viber is free for its calls, but does it allow you to make calls to number which are not on your list. You also mentioned it allows you to send text aswell, so im assuming you can also send images.

Just install it on your phone from Google play or itunes and see how it goes. Your phone will call those people using Viber who have Viber installed else the call will go through your carrier. I have noticed Viber's data usage per min is alsy very smart unlike some other voip solutions which hog your bandwidth.

Seems to be good, at least better than Skype. Will give it a try!

Someone mentioned there is also an application called Tango...whats this like?