Seemed limited to US networks but I'll add one anyway.
Other: Mobyson
I don't know if they have any presence outside Norway, or any of the nordic countries, but they're good.
Pre-paid only, but still very cheap, and very consistent pricing regardless of who you're calling (none of that "different cost for different network" nonsense), so it's ideal for me.
E-mail support only, but it's great and they're very quick to answer, so I don't care.
And I don't get the "pay to receive calls"-bit either. I remember seeing an explanation for it once, but I can't recall what it was.
I'm sure we have a couple of things that appears odd to americans though.