At the bottom of the page, Jona Lendering lists a biography of Mommsen in German by Stefan Rebenich, Theodor Mommsen, Eine Biographie. I wrote a review a while ago, and for German speakers, I can highly recommend it.
German Speakers may be also pleased to see that more works by Mommsen are available now in print. If you go here and click on the author's name, you'll get them all.
I've blogged Mommsen earlier also.
My own page with various links.