In Poland I used Plus, you can buy the sim anywhere, no passport needed. The sim I got is called 'Plus Internet na Karte', and for 10zl you get about 150mb of data. The sim costs 9zl and comes with 9zl on it. You can top-off in any gas station, convenience shop, they have some kind of machine, you tell the name of service and it prints out a receipt with a pin and you call a number and put the pin and it refills. I have forgot about 'APNs' so I didn't have data when I got there, the next day I went to a Plus shop (first day there was a holiday), and he set me up the APN and it works fine. Calls to the USA costs 1.80zl