Hamarsoft 86BUGS List - A reference for bugs and undocumented instructions in the Intel 80x86 family of processors.
Intel Assembler CodeTable 80x86 - Overview Mnemonics - Short overview containing all instructions (transfer, arithmetic, logic, jumps), a diagram of the registers (EAX, EDX, ECX, EBX), demo program, fits to one single page, download, free
Intel Processor Manuals - Official Intel Architecture Optimizations Manual, Intel Architecture Software Developer's Manual, Pentium Processor Family Developer's Manual.
Linux Assembly HOWTO - Guide describes how to program in assembly using free programming tools, focusing on development for or from Linux OS on i386. By Francois-Rene Rideau 1996-1999, Konstantin Boldyshev 1999-2002.
Lord Lucifer's Win32 Asm Page - Information on Windows assembly language programming that author have come across. It's primarily intended for beginners, but it should hopefully also be of use to more experienced programmers.
OOPWeb - Assembly Directory - Assembly programming tutorials, references and on-line books. All searchable and free.
Programming from the Ground Up - Book on Linux x86 assembly language programming which is available both in print and online under the GFDL license. Targetted to both new programmers and professional programmers who have not yet learned assembly language.