http://www.niehorster.org/
Massive compilation of WW2 orders of battle, tables of organization & equipment and administrative charts, all presented in a single graphical and informational style. Misses some relevant OOBs while including seemingly irrelevant ones such as Afghanistan 1939.

http://www.alternatewars.com/CARL/Nafgizer_CARL.htm
Last available download site for .pdfs containing thousands of orders of battle collected by a lone autist over the span of many years. Some of the files in the finding aid are missing, which likely means they're completely gone from the Internet.

https://www.feldgrau.net/forum/viewtopic.php?t=32002
Soviet OOB in Kharkov, 1943.

http://www.worldwar2.ro/
General resource on Romanian military history in WW2. Lists each division's regiments.

http://www.lexikon-der-wehrmacht.de/inhaltsverzeichnisGliederungH.htm
German site with information on each and every single Heer&SS division ever formed. Also has a lot of information on other topics.

http://www.diedeutschewehrmacht.de/
Information on each German army's composition over a wide range of points in time.