Depending on the duration of your stay, the data volume and bandwidth required, you could go for Tchibo, who offer a monthly data flatrate for € 20. SIMs are sold for € 1 at Tchibo shops and come with € 1 of initial credit. Tchibo uses the O2 network, which offers widespread EDGE coverage and HSDPA in urban areas. details:
Germany - Tchibo - Prepaid Wireless Internet Access
simyo and Blau, which both use the eplus network, offer a 1GB-pack for € 9,90 valid for month. eplus has recently started upgrading their GSM network to EDGE, but most of their network still supports only GPRS and their UMTS network doesn't support HSDPA. Also the eplus network is often congested especially in urban areas and so you may experience modem speeds even if UMTS is available.
Since simyo is only sold online, you will need to get a Blau SIM, which is sold at NORMA (supermarket), Kaufland (supermarket), real (supermarket), Media Markt (electronics retailer), SATURN (electronics retailer), ProMarkt (electronics retailer), Budnikowsky drug store), ateka (phone shop) and debitel (phone shop). Blau SIMs cost € 20 and come with € 20 of credit until the end of march - regularly the initial credit is € 10. For details on Blau check
Blau ? Prepaidwiki (German). If you don't subscribe to the 1GB-pack, Blau will charge € 0,24/MB.
The last recommendation would be Fonic, who also charge € 0,24/MB but also offer a dayflat for € 2,50/day, which needs prior activation by customer care. Fonic is also realized through the O2 network, which means EDGE and HSDPA coverage. SIMs are sold for € 10 including € 6,75 of credit at Lidl (supermarket), real (supermarket), Müller (drug store), dm (drug store),
JET (gasstation), ProMarkt (electronics retailer), KOMET (electronics retailer). Details:
FONIC ? Prepaidwiki (German)