Probably the best known dedicated mobile VoIP service, TruPhone works on Nokia E- and N-Series devices, BlackBerry phones, the iPhone and iPod Touch, and the Android-based T-Mobile G1. I tested it on a first-generation iPhone, using the handset's Wi-Fi connection. (You can use a cellular data connection on some handsets.)
After downloading the free Truphone app from iTunes, I was up and running in just a few minutes. The iPhone version retains the iPhone look--if not for the Truphone logo at the top, you might not even realize that the dialer isn't the iPhone's own. Calls to other Truphone users are free; the service also lets you access your Skype account (a boon for iPhone users waiting for the Skype client), and offers access to the major instant messaging services.
In my test calls, the voice quality varied greatly. Some calls sounded hollow, while others had a loud and distracting background hiss. But the service is usable, and its sound quality was far better than that of Skype Mobile.
Truphone has two pricing options: TruStandard, which has no monthly charge and rates that start at 5.1 cents per minute, or TruSaver, which costs $4 per month and has rates that start at 2.1 cents per minute.
Note: The download link takes you to the vendor's site, where you can download the version of the software that works with your supported mobile phone. Although the software is free, calling plans cost money.