Red Pocket Mobile is by far the best mobile company I've found. I live in the USA but call Canada constantly, and their rates are 10 cents per minute for calls from the USA to Canada.

They also allow me to dial directly from my cell phone to canada without any access numbers or other numbers, this is very important to me because I have 600 contacts in my phone and it would be very annoying if i had to first dial an access number, and then somehow remember the actual number i wanted to dial.

I've looked at many, many competitors, and the best rates ive found is 12 cents a minute, but at that rate, there is ALWAYS an access number to dial.

I got Red Pocket to call the Philippines, for which purpose they seem to have the lowest rates of any US mobile carrier. I used to use Virgin Mobile for this as they originally were the lowest (and before I knew Red Pocket existed), but their base rate kept creeping up and once Sprint bought them out, it was no longer cost-effective and I dumped them (they were not really competitive in the domestic market, either).
Ironically, at 10 cents per send/receive, texting on Red Pocket is half the cost of their parent network (AT&T) and probably lowest of any prepaid carrier or MVNO so far (this applies to both domestic and international text).
You can use any unlocked GSM phone on Red Pocket; it doesn't have to be limited to their offerings. While they may not make this clear on their website, you can buy just a sim from them for $25, which comes with $10 airtime, which is what I did and am able to use it in an unlocked mobile of my choice. (Call customer service to find out how to buy the sim).
I have had a problem dialing directly (011+country code+number) and called customer service regarding this matter. The nice girl told me what to do and although I followed her instructions to the letter, it did not work--fortunately, she told me about an alternative solution of an international direct dial number to use ("only if the first solution doesn't work"). I have to dial this number first, wait for the chime, dial the number I want to reach and then press #. So, I've had to use this method every time, as simply dialing the number directly still doesn't work. So far, so good; it's a bit of a hassle, and I hope there isn't going to be a problem with continued use of the IDD number (she made it sound like it was sort of a secret), but it's worked fine so far and call quality is superb (well, it *is* on AT&T).
Also noteworthy, their customer service is courteous and you actually get to quickly speak with a real live human being... during "business hours" (not 24/7 unfortunately).
The lack of round-the-clock live customer service and the fact that in my case at least, I have to use the IDD dial-through number with the added steps that entails, keeps me from giving them a full 5 stars, but they're still darn close. :) Probably the best prepaid mobile solution for calling Asia from the US that currently exists.