Google recently implemented Google Voice in its Gmail service, allowing your microphone-equipped computer (Mac or PC) to call any US or Canadian landline for free within your email. That's the best such service we've seen. (For comparison, Skype currently charges 2.1 cents/minute via pay-as-you-go or 1.2 cents/minute on a monthly plan for computer-to-landline calls.)Of note, international calls are also possible but do cost more. See Google's country calling list for fee rates.