Just found jajah.com, and brother, this must be the easiest way to use VoIP. Just enter your phone number, and the number you want to call to. Your phone will ring, pick it up, and wait for the other party to answer. That easy, no need for headsets, no download, no installation.

Rates are higher than Skype, and it always will, since they have to pay for two termination fees. And they are not all that competitive, see it for the portuguese market:

Note: all in network advantage is lost, so a call between two mobiles belonging to the same operator will also pay €0.35 per minute.

Update: Thanks to Joel for pointing Voipstunt, a VoIP service which allow free calls to land lines.