I used to do assembly language programming a while back, I was foolish enough to want to get back into it.
Back in the day, I used to compile asm code with MASM.EXE command line, writing code with no validation in a basic text editor.
What are the best tools in use today for writing in assembly?
What are some good quick references online?
My favorite assembler is FASM; it's multiplatform, up-to-date and it has interesting features (e.g. an easy and powerful macro engine).
The main reference is Intel's and AMD's documentation:
http://developer.intel.com/products/processor/manuals/index.htm
http://developer.amd.com/documentation/guides/Pages/default.aspx
You may like this cheat sheet as well: http://www.jegerlehner.ch/intel/IntelCodeTable.pdf
nasm:
The Netwide Assembler, NASM, is an 80x86 and x86-64 assembler designed for portability and modularity. It supports a range of object file formats, including Linux and *BSD a.out, ELF, COFF, Mach-O, Microsoft 16-bit OBJ, Win32 and Win64. It will also output plain binary files. Its syntax is designed to be simple and easy to understand, similar to Intel's but less complex. It supports all currently known x86 architectural extensions, and has strong support for macros.