What are your favorite philatelic links?

I've had one for 20 years, they are invaluable,  negatives, the list is immense, and dead links occur often.
There is one link that offers Philately books addressing years from 1500

My suggestion, would be keep the address to one line. with the cynosure text following in the same line

eg: "https://www.cse.psu.edu/~deh25/post/Timeline.html#1821-Table"  Great Britain Postmarks and locations
The cynosure text string must be brief and accurate .

 That way, a member can use the "Search" heading above, using the thread title,  and type in the box what you seek eg "Britain Postmarks"
that way members can search multiple pages in one click.

If the naming convention is brief and accurate, there is no need for categorisation, one can search for their discipline, in 200,000 listings with a simple CTRL F

Oops I see the problem  
Members cannot add to a central database, each member will have to add a post for every suggestion, or multiple thereof.

The above suggestion was how I use mine in Notepad

(1) I visit Postage Stamp Guide's Canada section quite often to look for information related to Canadian postage stamps:

postagestampguide.com/canada/stamps

(2) I also consult Stamp Raider's list of Canadian pictorial cancels that are grouped by themes:

stampraider.blogspot.com/p/canada-post-pictoral-cancels.html

(3) And unabashedly, I think the website on Canada's pictorial cancels that I administered is quite well done in filling up the information gap that several other sources have. I have several collectors who have helped me putting on the latest updates regarding these pictorial cancels. Although not everyone was able to provide high resolution image of their collection, the essentials are there. The site's associated Instagram account is getting noticed by Canada Post as well.

pictorialcancel.wordpress.com/

(As to why the site's web address doesn't contain the word 'Canada' -- it's because if I decide to purchase the domain for it later, the address will become something like pictorialcancel.ca. Putting 'canada' in it would be redundant.)

Anyone remember Joe Luft's Philatelic Links? He was one of the pioneers of collecting all the stamp related websites. I don't think it exists any more, but I found it on the wayback machine

web.archive.org/web/20080316225829/http://luftfamily.com/cgi-bin/newadditions.cgi

One of my favorites is the site of the “Stamp Friends Offenburg-Oberkirch”
The current new releases from Deutsche Post are presented here.
Includes publication date, Michel number and special cancellation.
briefmarkenfreunde-offenburg-oberkirch.iwebfree.de/cms/iwebs/default.aspx?mmid=3437&smid=4183

Helpful, for example in the “Mystery Stamps Identification Challenges”, if we are search for one of the newest German stamps that is NOT YET included in the catalog. ;-)

Impressive !   (Not tried any links)