iOS only: Gmail users can make free Google Voice calls, but why should they have all the fun? Talkatone sets up free VoIP calls, over 3G or Wi-Fi, from your iPhone, iPod, or iPad through Google Voice for free.
Talkatone does quite a few other neat things with your Gmail account and free voice-over-IP technology, but the main reason it's exciting is a feature that's somewhat tucked away. Double-tap on the Contacts header, and you'll get an option to make a Google Voice VoIP call. Choose a contact or dial a number, and you're calling. If it doesn't work on your first shot, you may need to do a little Gmail-to-Talkatone setup, described at the CyberNet News blog. Otherwise, it's a remarkably simple way to make a free Gmail phone call outside of Gmail.
Talkatone is a free download for iPhone, iPod touch (second generation and later), and iPad, and requires iOS 4.0 and later.