Vonage is not the cheapest. Skype is around $3 per month. Looking for something cheaper than skyper and that also supports SIP technology.
http://callingamerica.com provides free calls to US and CAN not sure if they use SIP, but it is a web based calling service, that you need to use a headset and mic with your PC to make the calls
http://magicjack.com provides unlimited US/CAN calls and uses SIP, there are some people who have figured out how to get it to work with a SIP device.. but it is against the TOS for the service, since they put adds onto the softphone on the PC screen - $50 for device and first year of service, $20 a year after that…
http://voipbuster.com provides free SIP calling to many destinations including US/CAN but does not provide you with a DID - but you can get a free DID at http://ipkall.com
http://gizmo5.com offers SIP calls to the US and CAN for 1 cents per minute…