Android Market

Sipdroid is an open-source SIP client for Android. This version supports VoIP over your Wireless LAN only (no voice over mobile data).

BigTinCan is a virtual calling card that allows you to make calls using your standard cell phone service at VoIP like rates. BigTinCan uses a callback system to connect your phone and number you are calling over the cell phone network, and you get a range of features including call recording, flexible caller ID and more.