Skype
I use Skype and can honestly say, it's great, maybe even fantastic!
It does depend on being able to get an internet connection - Broadband is better but it will work with a dismally slow connection such as 33kbps.
You can use a headset with your laptop (as I do) and there is even software to load on to your pda if you prefer.
I use it in hotels, airports, all over the place. It's dirt cheap - works out at 1.4p UK sterling wherever I am, wherever I'm calling. Credit is prepaid but seems to last forever. Pure Internet calls are free i.e. those not involving a terminating company.
What can I say...give it a go. I bought a plantronics headset that came with some free skype time.
I'm not a skype employee - just very impressed.