This research has been done on 2010-10-28; things will be different in the future.
The requirements have been these: something that does not cause any fixed monthly costs, but allows to access the German UMTS and HSDPA networks, from every location and in the highest speeds.
My recommendation is to combine the folowing rates, and always use the SIM card that is most appropriate for teh current network coverage and expected download sizes:
- klarmobil: offers a 19ct/MiB rate, which is probably the cheapest per-MiB rate available. Uses the O2 mobile network. No fixed costs. Uses HSDPA with 7.2 MBit/s where available. [Source]
- ALDI Talk Tagesflatrate: 1.99 EUR / day for mobile internet. Uses the E-Plus network, up to 384 kiB/s. [Source]
- fonic: 2.50 EUR / day surf flatrates in the O2 network, at most 25 EUR per month for mobile Internet, 3.6 MBit HSDPA (where available), GPRS speed from 1 GiB onwards. No basic / fixed monthly costs. [Source]
- simyo normal rates: 24 ct/MiB, E-Plus network.
- simyo 1GB package: 10 EUR to use 1 GiB of traffic within 30 days, with the option to more such packages if one did use them up earlier.
- fonic normal rates: O2 network, includes a 0.24 EUR/MiB data rate (but maybe only GPRS?), no fixed costs. [Source]
With this solution, one would (nearly) always have a SIM card to access an UMTS network that is available at ones current location )choosing from E-Plus and O2), and could also choose a rate that is adequate for what one wants to do in the Internet (like just checking mails, or surfing one day, or surfing for a longer time).