As usual, Google Voice can be found for free in the Android Market. Ah, and we must remind you to open the app at least once before routing calls through Google Voice. The app requires users to have an Android phone running 1.6 or higher-which is just about everyone with a phone running Google's open source OS.
Google Voice lets you place free domestic calls and cheap international calls and allows you to send free text messages.
source: AndroidMarket via AndroidCentral