Germany
---Names---
15th Century Low German Men's Names from Mecklenburg
http://www.ellipsis.cx/~liana/names/german/mecklenburg.html
German Given Names 1200-1250
http://www.s-gabriel.org/names/talan/germ13/
German Names from 1495
http://www.ellipsis.cx/~liana/names/german/german1495.html
German Names from Nürnberg, 1497
http://www.ellipsis.cx/~liana/names/german/nurnberg1497.html
German Names from Nürnberg, 1497
http://www.ellipsis.cx/~liana/names/german/nurnberg1497.html
German Names from Rottweil, Baden-Württemberg, 1441
http://www.ellipsis.cx/~liana/names/german/rottweil1441.html
Low German Names from Hamburg, 1475-1529
http://www.ellipsis.cx/~liana/names/german/hamburg.html
Medieval German Given Names from Silesia
http://www.s-gabriel.org/names/talan/bahlow/
Some Early Middle High German Bynames
http://www.s-gabriel.org/names/talan/Early_German_Bynames.html
Women's Surnames in 15th- and 16th-Century Germany
http://www.ellipsis.cx/~liana/names/german/womenssurnames.html
Women's Surnames in 15th- and 16th-Century Germany
http://www.ellipsis.cx/~liana/names/german/womenssurnames.html
German Names from Kulmbach, 1495
http://www.ellipsis.cx/~liana/names/german/kulmbach1495.html
---Clothes---
German Headresses
Both men and women
http://www.costumegallery.com/part1.htm
Hair & Headdress 1200s-1400s
http://www.costumegallery.com/part1.htm
Handbook of German Dress
http://www.costumegallery.com/germbook.htm
Handbook of German Dress
http://www.costumegallery.com/germbook.htm
---History---
German History
A quick summary of the German dynasties
http://www.germanculture.com.ua/library/history/bl_saxon.htm
---Language---
The Top 10 German Mistakes Made by Beginners
While intended for modern students, much of this will apply to the medieval as well. Just remember to compare this to your period research, don't use it as a base.
http://german.about.com/cs/grammar/a/mistakes5.htm
---Arts & Misc---