I have this new tutorial: http://www.skifactz.com/wifi/wifi_sip_phone.htm
While this is not a really a hack (or it is in a way because my phone was originally designed for Vonage service) it's still pretty exciting.
In the nutshell, you can use a mobile SIP compliant WiFi phone like Utstarcom F1000G to connect to any open AP and make free calls to other SIP phones. This costs nothing other than about $60 for a used phone. If you pay a few bucks more you can setup an account that allows you to call any landline at very low rates.