DOS and Windows

Binary Editors for DOS
With disassemblers EDCOM by Doug Cox and HexIt by Mikael Klasson.
http://www.bookcase.com/library/software/msdos.util.binedit.html

BORG
Windows 32-bit PE file disassembler by Cronos.
http://www.caesum.com/

GoBug
A Win32 symbolic debugger and disassembler providing output to the screen, file or printer.
http://www.jorgon.freeserve.co.uk/

Hiew
File viewer and disassembler for NE,LE,LX,PE formats and Netware Loadable Modules NLM,DSK,LAN, by Eugene Suslikov. [Shareware]
http://serje.net/sen/#hiew

HT
Is a binary file viewing and editing tool including disassembler for DOS and Windows 16/32 formats (MZ, PE, LE) by Stefan Weyergraf and Sebastian Biallas. [Open source, GPL]
http://hte.sourceforge.net/

Pass32 Assembler
For 32 bit protected mode/Windows applications, includes integrated disassembler, by Dieter R. Pawelczak. [Freeware]
http://www.geocities.com/SiliconValley/Bay/9159/pass32.html

PE Explorer Disassembler
A Windows 32-bit PE file disassembler and file editor, with a strong emphasis on peeking inside Delphi applications and packages.
http://www.heaventools.com/PE_Explorer_disassembler.htm

PEDasm
Is a Win32 command line disassembler targeted for win32 X86 platforms, by WITCZAK Nicolas Alexis. [Open source, GPL]
http://www.geocities.com/SiliconValley/Lab/6307/

Phoenix Disassembler
A Win32 PE File Intel 80x86 Disassembler, with API call conversion, and Navigation control. [Freeware]
http://www.minds.nuim.ie/~phoenix/projects.html#disasm

Programmers Heaven - Disassemblers
Dasm110, Desa101, Disasm32, MakeSrc2, Spy_Trak, Trace41.
http://www.programmersheaven.com/zone5/cat25/index.htm

simtelnet/msdos/disasm/
DOS based disassemblers.
http://www.simtel.net/category.php?id=73

VXDasm
A Visual disassembler for Windows95's VXDs devices drivers, by Jean-Louis Seigne. [Shareware]
http://www.chez.com/jls/vxdasm.html

Windows CodeBack disassembler
Windows 3.1 Disassembler by Leslie Pusztai Jr. [Shareware]
http://www.winsite.com/bin/Info?500000032266

Windows Disassembler
Windows 95/NT disassembler, by Sang Cho. Also Pentium II instruction table can be found at site. [Open source]
http://www.geocities.com/~sangcho/

Xope Systems
Offers Lantern (an advanced interactive decompiling environment in a form of a plug-in for the Interactive Disassembler Pro program) and reverse engineering services for x86 and compatible machine code.
http://xopesystems.com