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.