Talkatone makes use of Google Talk and Google Voice services. It is currently compatible with the iPhone 3GS, iPhone 4, 3rd gen iPod touch, 4th gen iPod touch, and iPad.
Android users will have to wait a while, but an Android version is definitely planned for release soon.
Basic Talkatone is free [ad-supported], but if you need support for more than one Google Account, or the ability to individually personalize contacts, a Talkatone Premium subscription [not free] is also available.
Talkatone [site link]
