Gizmo works just like Skype—free calls to other Gizmo users, cheap calls to landlines and mobiles worldwide. Unlike Skype it uses open standards (SIP), so it is compatible with other VoIP tools, such as Asterisk.
Jajah uses a different model—cheap or free calls between landlines or mobiles worldwide, set up from the internet. You can set calls up from any browser, or your symbian phone can set up the call via a GPRS/EDGE data connection or SMS message, giving cheap calls from your mobile to POTS numbers worldwide.
In any case, Skype may cost more than you bargained for.