As of November 2008, only two of these offer pay-as-you-go mobile broadband internet. They are T-Mobile and O2.
The idea of "pay-as-you-go mobile broadband internet" is that you use your laptop and a USB dongle to wirelessly connect to the internet. It works anywhere in the country that has cell phone coverage. The access is high speed and there is no contract commitment. You simply buy the dongle, plug it in, use the internet, then top up your credit as and when you need it.
T-Mobile's option is called the Xtra Pac T-Mobile web'n'walk Stick.
It costs a one-time fee of €69.95. For this you get a 77mm-long USB stick that can plug into your laptop, plus €10 of credit. The charge is then €4.95 per day of usage, midnight to midnight. Connection speed is 3,6 MBit/s. Download speed during the day reduces to 384 kbit/s after 1Gb, and 64 kbit/s after 2Gb. The stick can only be used with the SIM card that comes with it. If you want to use another operator's card you have to pay a €99.50 unlock fee or wait 24 months.
O2's option is called the O2 LOOP-Surf-Stick.
It costs a one-time fee of €69,99, similar to the T-Mobile option. For this you get an 87mm USB stick, plus one month of credit. Rather than paying day-by-day, O2 has month-by-month options. The €5 pack buys you one month of access limited to 30 MB and the €10 pack buys 200 MB. The €25 pack has no hard data limit, but connection speeds are reduced after 10 GB.