I don't have a reference in English, but http://de.wikipedia.org/wiki/Impressumspflicht (in German) is very clear about it.
Wie sich aus § 55 Abs. 1 RStV ergibt, trifft
einen Anbieter somit nur dann keine Impressumspflicht, und
er kann seine Webseite völlig anonym ins Internet stellen,
wenn sein Angebot ausschließlich persönlichen oder
familiären Zwecken dient.
The problem with this law is, that is says something about a page with some baby pictures ( so that grandma can see her grandchildren), and something about media corporations. However it simply does not address the case of a private blog, were the audience is not personally related to the author, but the author does also not try to make money with it.
A blog, a portfolio, bookmarks, dotfiles or even a simple memorial (if it's for someone outside of family) is enough.
I have not added one to anything I have ever put online nor do I intent to. I neither host in my country nor do I publish in my mother tongue. I think it is ridiculous to be forced to divulge information that allows people to knock on my door.