Found this on Onesuite blog. Comparison of the four most popular free softphones in the market. Just in case you need to know what softphone is, its an application for VoIP service. Most VoIP service can be use on softphones when an ATA device can't be use (say while at the airport or coffee shop) So you can make VoIP calls in your laptop while waiting for your flight (most airports has free wifi) or even while on the road using an aircard (for internet access)