There is a lof of free and not really free VoIP clients for Symbian Nokia and Sony Ericsson devices like BaraBlu, AQL, Mobiboo, Skype, RebTel, Jajah, Truphone and possibly others. But we talk now about really free software clients that allows to use SIP gateways on Symbian 9 and Symbian 8 smartphones. You always can use special edition of Nokia smartphones Ineternet Edition - N80 IE and others. These devices have embedded SIP client that can be configured to work with any SIP gateway server. But what to do with other Symbian smarphones? We need to install some free SIP client on the smartphone. One of these applications is Fring. Since version 3.0 it allows you to call not only to Skype contacts, but configure a SIP service and directly use it. Now you can call for free to many countries with SIP services like Gizmo, Free World Dialup or bunch of Betamax services like VoipStunt, VoipDiscount, VoipCheap, NetAppel, Voipbuster, SparVoIP etc.
Update: Read about new Free Skype VoIP client in Nokia phones, it's ready to be released in Nokia N97 phone