Michael Duffy's First World War.com is an invaluable compendium of the First World War. Absolutely brilliant. Not to be missed!
The Library of Congress website - no further comments!
Small, but important site with a crash course in history of Poland and an interesting guide to Krakow (Cracow).
Only a single page site (apparently under construction) but there's something touching about it. Visit it if you are interested in the Holocaust.