In the following I have listed selected links to websites I found helpful for me or of general interest:
nikonrumors.com
If you can't wait for the official announcement of a new camera or a lens take a look here. In most cases you'll find the main facts about the item weeks or months before. But this site is more than just a rumors site: it gives you important information about new accessories (Nikon and third party), too. And you'll also find links to reviews and reports. For me, nikonrumors.com is an important site I visit regularly.
(nikonlinks.com)
Update September 28, 2014: nikonlinks.com seems to be finally dead. A pity!
lenscore.org
senscore.org
These two Swiss sites together are a very good and less commercial alternative to dxomark.com. You will find a ranking of full-frame lenses (lenscore) and one of the sensor quality of DSLRs (senscore).
bythom.com
On his website Thom Hogan provides a lot of helpful information about photography and in-depth reviews of Nikon gear.
photographylife.com
You will find a lot of well-founded reviews of lenses and other gear on this site. I find the endless blogs at the end of the articles a bit boring and would suggest skipping them.
naturfotograf.com
Bjørn Rørslett is a Norwegian professional nature photographer. Today he is one of the operators of nikongear.com. For me, his - still available - legacy site has been the most helpful site ever!
kenrockwell.com
Ken Rockwell seems to be a professional amateur-photographer ;-) His very helpful website contains a lot of practical information. You'll find tons of reviews, even of older gear.
On the other hand, in my opinion, Ken Rockwell has a tendency to simplify things.
photozone.de
This site contains a lot of standardised lens reviews. Very helpful!
momentcorp.com
Frederik Rasmussen offers a lot of reviews, even of older lenses, on his website. Have a look!
mir.com.my/rb/photography/companies/nikon/htmls/models/index.htm
Beautiful site! You'll find photos and descriptions about nearly everything Nikon has ever produced for the F-Mount.
photosynthesis.co.nz/nikon/
The website of Roland Vink delivers a lot of information, especially about serial numbers. Very helpful if you want to buy older gear.
braczko.de
Peter Braczko is the author of the 'Nikon Handbook', which is still today an important source if you buy older used equipment. His website is under construction...
butkus.org/chinon/nikon.htm
On this website by Mike Butkus you will find a lot of manuals of older Nikon gear in PDF format. Great!
legacy2digital.com/nikon/
Legacy2digital - located in the US - offers AI conversions and CPU conversions ("chipping") as well. On their website you'll find a lot of detailed information about their conversions. They also sell already chipped lenses.
Be careful with chipping a pre-AI-S lens!
lezot.com
LeZot Camera Repair offers AI conversions. Chipping is also offered, but they give no further information about that.
aiconversions.com
John White offers AI conversions. I recommend you to take type B! Moreover he offers the mounting of an old style meter coupling shoe on series E or AF lenses.
schneideroptics.com/info/stillphotography.htm
On schneideroptics.com you'll find a lot of information about B+W filters. The link above provides some downloads, among others a PDF with transmission curves of B+W filters.
http://www.schott.com/advanced_optics/english/download/index.html
The link above provides a lot of downloads, especially a helpful PDF ("Optical Filter Glass - Properties - 2013") with transmission curves of Schott filters.
Every now and then, B+W produces a batch of filters with Schott glass. Ask your B+W dealer. For instance, I own a B+W filter with Schott BG-38 glass.
http://www.optic-makario.de/
Optic Makario GmbH is situated in Germany. They are specialised in modifying digital cameras for IR and UV photography. Over the years they have modified three DSLRs for me. They do a great job! But it would be a good idea to publish more detailed information about their conversions. They are much more expensive than their American competitors, see below.
http://www.lifepixel.com
http://www.spencerscamera.com
http://www.precisioncamera.com/infrared-conversion-services.html
These are three American shops which offer IR conversions. They appear reliable to me, but I have no personal experiences with them.
http://photographyoftheinvisibleworld.blogspot.de
On this site from Dr Klaus Schmitt you will find an in-depth analysis of all aspects of UV photography.
ewa-marine.com
If you are searching for a cheap solution for getting your DSLR under water I recommend the flexible housings from ewa-marine!
Maybe the flexible housings from "DiCAPac" are an alternative, but I don't have any experience with them. See here: dicapac.com
home.swipnet.se/marinls/domeuw.html
This link leads you to a very good article about the problems of underwater wide-angle photography, flat glas, dome ports and so on. It was written by Leif Samuelsson from Uddevalla, Sweden.
VueScan
Don't throw away your old scanner because it is no longer supported by the manufacturer and/or your operating system. Have a look to Ed Hamricks "VueScan" first. This great and cheap scan-software supports a lot of scanners, even older ones.
CombineZP
CombineZP is a great stacking software. It's freeware! Although it looks and feels a bit old-fashioned, the algorithms work very well.
Fisheye-Hemi
Fisheye-Hemi is a photoshop plug-in that corrects fisheye distortions only in vertical direction for images in landscape orientation.
Irfanview
A versatile image viewer - my favourite for many years now.
If you want to use Irfanview on two or more monitors you need additional software, see my article "Presenting images on two monitors".
On one computer and two monitors AutoHotkey is a good addition.
On two computers and two monitors I recommend Input Director as an addition.
WildBit Viewer
The WildBit Viewer is a very good image viewer. It is able to serve two monitors - one vertical plus one horizontal without any preparations!
zoomyrentals.de
If you are located in Germany and you are looking for an online rental service I can recommend Zoomyrentals. They lend gear at attractive prices. I used the service once, it was a very smooth transaction: I rented an AF-S Nikkor 300mm f/2.8G VRII for a week. The lens came by express shipping. It was in a very good condition. A return label was included. After the week I just had to put the lens back into the package and wait for the express pick-up service.
froknowsphoto.com
This is the website of the leader of the 'I shoot raw' campaign, Jarid Polin. There are a lot of videos about photography, amongst others a virtual discussion with Ken Rockwell about raw vs. jpg (Rockwell shoots jpg). A shop where you can buy t-shirts, combs and other important things is included too. In my opinion there is more show business than serious information. Nevertheless it's funny.
