If you guys want serious foam, buy the camspray foamer. This is the only foamer readily available in the US which works well. All the others are really just a waste of time. It's relatively expensive, but that's the cost of foaming! With a Karcher, you'll have to buy the adapters mentioned previously in this thread also.
Here's the link to the foamer:
http://www.everypressurewasher.com/C...0-CAM1243.html