Overview

Blau.de is an operator on the Eplus network, the company offers their services only online on http://www.blau.de and has about 600.000 customers (2007).

Price

The prices are changing every few month or even within weeks, special offers are common. In May 2009 calling is 9ct/min to all German networks and 9ct/SMS in Germany/20ct to any other country, Blau.de also offers a prepaid calling flatrate for 15€/month.
For data connections 24ct is charged per MB, counting in 10kB blocks unless you spend 9,90€ on a 1GB datapack or 19,90 € for an Dataflat.
The SIM-card costs 20€ including 20€ prepaid balance.

Where to buy

Order the SIM-Card online, you will need a valid German address and about 4-5 days time until you receive the shipped card.

I have seen blau.de simcards at Saturn (chain of electronics/CD/DVD/telecom stores), but they were hidden inside a counter.

How to buy top up credits

One way is to setup automatic loading by credit card or bank collection, you can set e.g. a minimum limit of 10€ on which it's going to load the account by 15€.
You might also login and purchase credits by credit card or bank collection manually.
In case you can't get online you will find top up cards in over 6000 Shops throughout Germany such as, gas stations (AGIP, Shell, Aral and others) , kiosks, specialized trade stores and food shops.

Activation

Card is activated on first call.

Misc

Eplus UTMS coverage is not good in rural areas, as fallback the Eplus network only has EDGE capabilities in large cities, otherwise you only have simple GPRS which is GSM.
APN internet.eplus.de, if you need to dial a number use *99***1#, username: eplus, password:eplus

Alternatives
Congstar (T-Mobile Network)