I second Dilbert's choice of VoiceNet. They offer 6-second billing and no connection charge. Rates to Russia, etc. were better than from my office phone!
As a caveat, I have had occasional customer service problems with them. Despite repeated calls to customer service, my account won't let me call back to the US from overseas. I think it's an account-specific problem, though, as a colleague of mine who travels internationally far more than I swears by VoiceNet.
I have also had bad experiences with VoiceNet's 1-800 customer service domestically--it once took me 30 minutes to get my card reactivated to make a 2 min call after they had 'accidentally' deactivated my account, and I had to request credit for fraudulent calls 3 times (but on the plus side they initially spotted the calls and contacted me; evidently the crediting problem was because the original call to me was not properly handled by the proactive employee).
Despite these seemingly negative experiences, I still recommend VoiceNet. The six-second billing, reasonable rates, and clearly itemized invoices are all worth it. And if a competitor offers better rates, ask for it--I threatened to cancel after my customer service problems, and on the spot they dropped my domestic rate $.026/min (currently $.149, with no connection fee).
Try
http://www.voicenetcard.com.
[This message has been edited by chalf (edited 12-28-1999).]