Skype as we all know, is the largest VoIP provider in the world.
Pros: Signing up for skype is as easy as ABC. It runs on most Window system. It claims that it is a secure communication because users are not involved in the encryption process.The huge plus point for Skype is, it provides free PC to PC call even though we are continents apart!. I found that the audio quality is more than superb, just like talking to a person next door even though the other person is in another part of teh world.
Cons: Skype signup does not need any proof of the identity of users. Hence users can use the system without identifying oneself or they can claim to be another person. Naive users can easily be tricked if another person use the personal name of a person the user trusted and trick the naive user to provide private informations. There is lack of privacy since Skype has the keys to decrypt calls or chat sessions so we need to be more careful what we tell the person at the other end.
Additional thoughts: I sill love Skype because it helps to cut down my telephone bill tremendously.