On the T-Mobile network there are only Penny Mobil (sold at Penny supermarkets) and Rewe Mobil (sold at Rewe supermarkets), who both offer a dayflat for € 2.50, but bandwidth is throttled to 384 KBit/s, allthough T-Mobile's 3G-network is completley HSPA-enabled. These dayflats are billed by calendar day (from 00:00 - 23:59 h).
Choice is wider on Vodafone:
Vodafone themselves offer the so-called "Websessions", that can be used with any prepaid SIM from Vodafone Germany and can be paid by credit card at the beginning of each session. Pricing is:
7 days (limited to 1GB) for € 9.95
24 h (limited to 1GB) for € 4.95
2h (limited to 1 GB) for € 1.95
15min (limited to 1 GB) for € 0.49
There are two MVNOs, that offer Vodafone's Websessions with modified tariffs under their own brand:
BILDmobil (named after the German tabloid "Bild")
7 days (limited to 1GB) for € 6.99
90 min (limited to 1GB) for € 0.99
30 min (limited to 1GB) for € 0.59
N24 mobil (named after the TV channel "N24")
7 days (limited to 1GB) for € 9.99
12h (limited to 1GB) for € 2.99
1h (limited to 1GB) for € 0.99
You'll find detailed information on Vodafone's Websessions at
Germany - Vodafone Web Sessions - Prepaid Wireless Internet Access
BILDmobil's and N24's offers are absolutely identical to Websessions (including APNs) but tariffs differ and they are only distributed online.