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.
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.