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 - FAQs: A fairly detailed discussion of how one commercial disassembler works.
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.