Documents:

Intel Code Table (PDF)
Art Of Assembly 16bit (Zipped PDF set) [MUST HAVE!]
Art Of Assembly 16bit (Zipped HTML)
Pentium Pro Family Developer's Manual:
- Volume 1: Specifications (PDF)
- Volume 2: Programmer's Reference Manual (PDF)
- Volume 3: Operating System Writer's Manual (PDF)

Compilers:

NASM (Download page)
FASM (Download page)
MASM (Download page)

Other:

HelpPC 2.1 - Provides assembly instruction, interrupt and hardware data lists. [MUST HAVE!]
Ralf Brown Interrupt List (RAR) - Big list of interrupts and instructions.
Ralf Brown Interrupt List (Online HTML, no instruction list)