After getting gouged for a while by MCI, I switched over to Primus, a provider here in the UK. It is 6 pence (about 10c) per minute back to the US from the UK and not that much more than that from the rest of the world. They keep a balance of your funds and each call debits off that account. Sounds similar to the IDT card discussed above, but with better rates from the rest of the world.

I set up the deal here in the UK, so I am not sure what their presence is in the US.

www.primustel.com