iOS only: Talkatone, the rather nifty app that allows free Google Voice calls on iPhones, has updated with a a much friendlier interface, access to your phone's own contact list, and better connection and audio compression handling. It also works with capable iPod touch and iPad models, too.
The main improvements are on the backend, involving audio quality, call monitoring, and other network and voice technologies. But making a free call through Google Voice is no longer a function you have to hunt down in a tap-to-open menu, and you'll get notifications about incoming calls when you keep the app running in the multitasking dock. If you like to save minutes on your cellular account, or if you're often in locations with decent Wi-Fi but not such great cellular service, Talkatone is a great option.
Talkatone is a free download for iPhones, iPods, and iPads. A Premium version unlocks certain other features, including better compression for calling.