Gmail / Google Voice: Free Internet Phone :

Google Voice enables centralized telephone calls on a single number and free calls (in some cases).

Google Voice has many features: free phone call for national (paid abroad), send free SMS, divert calls to a landline or mobile messaging to a conversion of audio messages (telephone conversations or messages voice mail) in text emails ...

GrandCentral already allowed to make free telephone calls in fixed, mobile and VoIP on a single number. Voice adds the following features (not exhaustive):
- Transcription of audio conversations (Google Voicemail)
- Support for SMS
- Conference Call
- Integration of GOOG-411
- Low cost international calls (payment via Checkout)
- Integration with the Google Account and in particular to address book
- Block matching
- Recording of telephone conversations
- Consultation (old) transcribed telephone conversations via the Internet or by phone

So far Google has not been authorized by Apple to make available for iPhone Google Voice.

SIPDroid free phone with Android :

Sipdroid client is a SIP / VOIP for Android to make and receive telephone calls over the Internet using, for example, your SIP account Free.

Some operators block VoIP software on 3G. We must therefore pass through WiFi. SIPDroid is an application that you find on the market Android, or on the website SIPDroid.
To run the software you need to register with a SIP provider like pbxes.org (free).
If you have a freebox, it will go on your account and under phone, activate the SIP.
To configure your account pbxes instructions should be followed (in French) SIPDroid wiki site.
The new version of SIPDroid allows video calls.

Finally, remember that you can use the software to find WiFi Freebox Free WiFi hotspots and make calls and with Sipdroid.
Here is the procedure:

To activate your Free WiFi at home, here's how:
1) Place front of your Freebox
2) Put yourself in WiFi. Launch Internet and enter the address and watch your http://wifi.free.fr Freebox, it will display a four digit code that will then enter on the form
3) Confirm and enter a password of your choice

Site SIPDroid