Buzzroam (Level 19, Two International Finance Centre, 8 Finance St. Central, Hong Kong, China; phone, in the US, 415/829-5219) offers global SIM cards with attractive rates.
Examples — calls from France to the United States cost 41¢ per minute, while Mobal.com charges $1.36 per minute. Calls from China to the US cost $1.36 per minute with Buzzroam as opposed to $3.59 with Mobal.
Instructions are given on how to forward your mobile phone number in your home country to ring your prepaid SIM number.