That's right...with smartphones all that was possible using computers is increasingly becoming feasible through phones. I just now configured my Nexus S to make free phone calls over internet to USA. How you ask? Well here we go:
The site offering free calls to USA landlines and mobiles is www.voipstunt.com so I registered an account with this site.
Now Android 2.3 has inbuilt SIP support so I directly configured instead of downloading any app from market but it can be as easily done using sipdroid or csipsimple but anyway since there is good dialer integration I opted for direct route.
This is it. All done. Try calling landline or mobile number of your US contact and before placing the call your phone will ask whether to place call over internet or cellular network. Choose internet and you are off to free calls.