uboat.net
uboat.net is the absolute best English language source for
uboat history to be found anywhere on the web.
It mainly covers the Second World War, however it has a
growing First World War section.
There are large numbers of experts, historians and authors
on the forums there who frequently go out of their way to
help the amateur enthusiast.

www.u-35.com
This is a first rate site by Hans Mair. Hans has been researching
the history of a single boat, U-35. Hans leaves no stone unturned,
and sets a high standard.

www.uboatarchive.net
A site with highly detailed records of destruction of a few uboats,
interrogation reports, and excellent quality archival photos.

More photos by David S Speaks
David photographs Armor, Ships and Aircraft. His clear, well chosen
views of Military equipment are of great interest to the enthusiast
as David knows exactly what we want to see.

German language sites

uboot archiv
A fantastic archive, and the web site
is improving constantly. Much information available, but you must
understand German to get the best from it. Non German speakers will
still be able to click on various links and find interesting pages.
The forums there are good too but 99% German language.

u boot greywolf.de
Lots of good articles - for the German speaker only though.