Not only is the Vencall app free, it can also save you a whole stack of money. It's advertised as being a great option when travelling as you can avoid high roaming charges, but this needs to be taken with a pinch of salt. This is true only if you can find yourself a free Wi-Fi connection, otherwise data charges used getting a suitable connection would end up costing you a lot more when overseas than a call would be, so this is something to consider.
You need to sign up for a Vencall account, but again this is free. Simply enter in your login details once you've installed the app and you are good to go. There is also a huge list of countries where Vencall can be used -- you'd be hard pushed to find a common destination that isn't included.
The voice quality can be a bit hit and miss, but this is largely down to whatever connection you are able to get, which can be said of any VoIP service. There are also apps for Android and iPhone, so you can use it to connect with your friends if they aren't fans of the BB.
The calls aren't free, but you're talking super low cost -- the average being 2-5 cents a minute.
If we've left out one of your favorite calling apps on BlackBerry then please leave a comment below.