Well, they aren't travel or folding keyboards, but I've always been a big fan of the USB versions of the IBM UltraNav keyboards.

They have been available on ebay lately in non-English variants:
( Swedish Ultranav on Ebay) (Though I think the price on that one is already a bit too high)

The compact models (not the ones with a num-pad) are relatively thin and light. Pre-N800, I traveled with one of these keyboards, a flash drive, and a Knoppix CD instead of a laptop.

I haven't seen much in the way of plain USB portable keyboards. I looked around for an Adesso, but the models in stock at various online retailers always seemed to be ones with proprietary PDA connectors.