I have looked at the o2online.de website and found the correct information about the two tariffs that I mentioned earlier in this thread.
"O2 on": an airbag + Handy-Surf-Flatrate + Internet Packet M + rebate.
Cost 50 euros per month. 24 month contract. Up to 200MB @ 7.2 Mbit/s, then unlimited gprs.
"O2 on Upgrade": an airbag + Handy-Surf-Flatrate + Internet Packet L + rebate.
Cost 65 euros per month. 24 month contract. Up to 5GB @ 7.2 Mbit/s, then unlimited gprs.
For companies, the business equivalent tariffs are called: "O2 on Business" & "O2 on Business Upgrade".
Alternatively, for people who only need a small but free internet allowance, there are ad-financed "netzclub" sims with an official 200MB monthly quota (but reportedly 374MB in practice). A big thumbs-up to Telefonica for this new service!
NetzClub - Zahlen können Andere