Free Internet calling services for cellphones.

Even if the possibilities have been around for a while, making free international calls via cellphones have remained challenging. Two companies, Skype, the market leader and Fring, an Israel based company, offer the most user-friendly cellphone applications for international calls. But, there are still some hussles and for these applications to be worthwhile, someone has to plan to make a lot of international calls.
For these two applications to work best, they better be downloaded on a phone with WI-FI capabilities with the user staying within the hot spot during the phone call.
For both Skype and Fring, users are assigned an ID or name that they use to dial up with the call going through the service's Internet servers.
Calls made to others members of both services are free while those made to non members using a Skype prepaid account, are at a cheaper rate than the one charged by traditional phone companies.
The Skype cellphone application can be dowloaded directly to a phone using Windows Mobile or to a PC and then transfered to the phone later.
According to the New York Times,"for those without a Windows Mobile phone, Skype recently introduced Skype Lite, which runs on dozens of Nokia and Samsung phones with Symbian software, as well as a few Motorola Razr models. On Skype.com, these users can type in their mobile numbers and Skype will send a message to the phone with a link to download the software."
Skype Lite also runs on the G1, from T-Mobile, also known as the Google phone, which operates on the Android software platform. To get the application,one's only need to visit the “Market,” Android’s app store and click on the free Skype application.

For more, see NYTimes.com