Yeah, Skype is the one you want. It's in beta but the major bugs seems to have been worked out, it works well and allows you to call landlines/mobles at pretty good rates too. Don't forget iChat AV also has VoIP capabilities.
The best softphone for mac is X-ten's X-Lite app, as for making calls out try gossiptel.com, telappliant.com if your in the UK and vonage, stanaphone or Sipphone.com for US.