PC Assembly Tutorial
Ragestorm - Programming World
386intel.txt INTEL 80386 PROGRAMMER'S REFERENCE MANUAL 1986
387intel.txt INTEL 80387 PROGRAMMER'S REFERENCE MANUAL 1987
Intel Developer Zone - Introduction to x64 Assembly

flat assembler is a place dedicated to assembly language programming and contains many resources for both beginners and advanced assembly programmers. Also offers the open source flat assembler assembly compiler.