For those who have relatives abroad, keeping in touch with them in real-time is so important. And for businesses also that rely on Skype esp. on making out-bound calls, it is also important not to miss out every client or it’d be a big loss for the company. But why should you rely only on Skype when you could have many options? Here are the top ten Skype alternatives (though I want to suggest Yahoo messenger also):
1. ComBOTS – connects your PC or landline with others around the world for free.
Pro: Rich-media IMs with audio & video
Con: Can’t make mobile calls
2. Gizmo Project – Open-source VoIP softphone that offers free or minimally expensive calls worldwide
3. Google Talk – Available only in the US (I did try this but I was not allowed to.)
4. iChat – as excellent as Skype
5. Jajah – offers free PC-to-PC VoIP calls and a limited number of free calls to and from landline phones
6. WengoPhone – open-source app that’s developed through OpenWengo.com
7. ooVoo – allows you to videoconference with up to 6 people at once and to send out video messages rather than emails
8. SightSpeed - free PC-to-PC voice calling and videoconferencing, and its $4.95-per-month Pro service adds video-mail recording and multiparty videoconferencing
9. Vbuzzer – offers PC-to-PC voice VoIP-based calling, faxing and videoconferencing for free, with nominal rates for completion to landlines and mobiles worldwide
10. VoipBuster – allows free calls – for a price
For more details, please check source.

Yahoo! Voice is about 2 cents per minute in most places.

There is VoipCheap as well, with several countries that are free calling destinations.

Here are some others you can check out (but there are more):

Evaphone
iCall