With fring call locally or internationally at a cheap rate using SIP. Easy to download and simple to install. Use it with mobile that has wifi access and 3Gcapability.
Fring only works with Nokia phones which use the Symbian operating system. It will soon be available to Windows Mobile 5.0 and Windows Mobile 6.0
Free Download http://www.fring.com/download/
Latest update:

fring for iPhone
See details here: http://www.fring.com/download/iphone/

Gizmo5 Mobile

Only through SJPhone. Available to Nokia cellphones that has WLAN enabaled handsets, Blackberry. Uses Windows Mobile operating system.

Free Download http://gizmo5.com/pc/products/mobile/

Nomado

Softphone for Smartphone or PDA
Only through Nomado Telecom .
http://www.nomado.eu/Download-softphone-free
iNomado for iPhone to be out some time in July 2009

Jajah Mobile Web

Available only after creating Jajah account.
Supporting only Java based (J2ME) and Symbian TM S60 version 7.x or 8.x phones
http://www.jajah.com/products/mobile-web/

Nimbuzz!

Offers 3 types of calling options: Dial-Up VoIP, VoIP and SIP
Supporting Java, Symbian, Windows Mobile, iPhone O.S[5], and Blackberry.
http://www.nimbuzz.com/en/mobile/download

Truphone

Operates on Symbian software, iPhone, iPod touch

http://www.truphone.com/applications/devices/nokia/download.html

Vopium

iPhone, BlackBerry, Android, Symbian OS[10], Java (programming language) and Windows Mobile[11]
https://vopium.com/get-started