If you want a link adding here, then please let me know
Phoenix Disassembler Only on version 0.5 at the moment, but a disassembler which is looking good so far.
PlanetBooks Free online books in just about every computer subject you could want to read about, at last view it had 680 books on there. An excellent resource, and will save you some money - so now you owe me :)
Decompilation Page Well worth a visit. This page contains links to many other disassemblers and to various technical resources on Disassembly and Decompilation.
Sandpile This is a good site for information on processor instructions, both documented and undocumented.
Ralf Browns Interrupt List A resource for the old DOS programmers, Ralf Browns interrupt list is the definitive guide to interrupts on the PC.
Wotsits File Formats Quite a few people have been asking about the format of the PE file. Wotsits file formats is a good place to start looking for information on file formats, and provides a good set of resources on all kinds of file formats.
www.MyFileFormats.com. I have actually removed the link to this site. They just blatantly copied the whole of the wotsit.org site and called it their own. Although all of the information is public domain it's something that just is not done. I urge you not to use this site, and to use Wotsits instead, a site which someone has put a lot of hard work and effort into providing good resources for programmers.
Intel Manuals for the Intel architecture manuals and detailed information on instructions,etc.
Pentium Optimisation by Abner Fog is another interesting site for optimisation. [The pentopt.zip file contains an excellent article on pentium assembly code optimisation which first introduced me to the concept of code stalling when I was having some optimisation problems with some code - Cronos].