I am a fan of Skype for its free PC-PC calls. On the whole, great quality and combined with a camera thru an IM program it's great for calling home. However the need to have a PC has always been a restriction.

Now if you have a PDA with WiFi you can make calls at an increasing number of spots with Skype Zones for a low monthly fee or a one-off charge. So you then have a mobile phone which connects cheaply to another Skype user or at low rates to the regular phone network.