Sommerfeld Family Forum: a blog (in German) organized by Dieter Sommerfeld.  It is a wealth of information on Sommerfeld's from all over the world.

Familienforschung in Westpreußen: a website (in German) with information on how to research family history in the former West Prussia.

World names - Public Profiles:  A website that shows the modern day frequency of surnames across the world.  The Sommerfeld name is concentrated in pockets.