Clive Turvey's Home Page
Source for a commercial disassembler and a selection of free downloads related to disassembly. Also has a useful collection of links to related sites.
COBOL missing source code recovery, disassembler -
A fairly detailed discussion of how one commercial disassembler works.
Documentation for disassembler writers
By Ilfak Guilfanov the author of The Interactive Disassembler (IDA). This page offers a collection of executable file formats.
Giampiero Caprino's Home Page
Contains useful information about Decompilers and File Formats.
Object Code Translation Home Page
Some introductory discussion of disassembly. Also has a glossary, and short description of file formats. Many links to other resources.
The Decompilation Page
And in depth discussion of the theoretical issues involved with disassembling programs.
Transform / DeCompilation
The new site maintained by Cristina Cifuentes. An in depth resource for program disassembly information.