http://web.tiscali.it/scambiosoft/
Iczelion's classic site which contains a large amount of reference material, assembler example code and links to other assembler resources.

http://webster.cs.ucr.edu/
Randy Hyde's main site contains a wealth of useful information for assembler language programmers.
It also features the HLA (High level Assembler) project and the classic Art Of Assembler documentation.

http://www.madwizard.org
Thomas Bleeker's popular assembler snippet collection.

http://www.assembly.com.br/
Ketil Olsen's very popular assembler IDE has its own homesite and support forum where you can download RadAsm and a range of useful accessories for it.

http://www.massmind.org/images/www/asmedit/AsmMain.html
Ewayne Wagner's ASMedit IDE can be downloaded from this site. There is a comprehensive range of accessories for it on the site as well.
 
https://winasm.org/index.html
This is the home site for Antonis Kyprianou's WinAsm Studio IDE. It is part of a support forum that Antonis has set up to support programmers using his WinAsm Studio IDE.

http://www.agner.org/
Agner Fog's work on assembler code optimisation is a must for any assembler programmer who is interested in performance. You can download his manual in a number of different formats from the assembly programming section on his site. You will also find advanced random number generation algorithms written in assembler.

Test Department
TD has announced that he is closing down his project so to preserve his excellent work for programmers interested in well written MASM32 assembler code.
His complete archive has been preserved so that new programmers can learn from it.

https://www.intel.com/content/www/us/en/developer/articles/technical/intel-sdm.html
The Intel manuals are the best available reference material for the various Intel processors. They are a  "must have" for assembler programmers.

http://www.amd.com/us-en/Processors/DevelopWithAMD/0,,30_2252_875_7044,00.html
This is the link to the AMD site for their 64 bit series of x86 processors.

The SpAsm home site
This is the home site for a new assembler being developed. It is fully independent of any existing assemblers and is distributed under the GPL licence as open source software.
Betov has a reasonable amount of reference material available for programmers who want to investigate a completely new and different assembler.

http://www.rbthomas.freeserve.co.uk/  A current site is unknown
Ron Thomas is an assembler programmer with years of experience, particularly in the area of graphics programing. You will find a wealth of 32 bit MASM graphics code examples on his site of differing degrees of complexity. To build Ron's examples, you will need to use NMAKE unless you know enough about MASM to build it with a batch file.