Best FOSS alternative for skype?

[Marko Vojinovic]

Ok, I just lost enthusiasm in further use of skype. Being closed source was 
bad enough already, but I tolerated it due to its good-enough quality, 
reliability and free-of-charge use for skype-to-skype calls. But now that 
Microsoft is taking it over, I suddenly lost faith that these good features 
will still be there from now on (not to mention Linux support).

Thus the question: is there a FOSS VoIP app that provides roughly the same 
quality, reliability and free-as-in-beer service?

I do know that there are such VoIP apps, but I am not familiar with the 
quality of service, availability and accessibility of each of those. I am 
willing to migrate and start persuading my existing skype contacts (which are 
scattered all over the globe) to migrate to a FOSS solution as well. But I 
cannot do that if this alternative does not provide functionality that at 
least matches that of skype. It has to be up to the task.

So which one is on the top of the list, in your opinion?

TIA, :-)
Marko