Windows API reference
Description of Windows API functions.
URL: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/winprog/winprog/windows_api_reference.asp

Linux kernel syscalls reference
Description of linux syscalls, with includes for FASM and gas.
URL: http://lscr.sourceforge.net/

Single Unix Specification v3
POSIX standard for unix-based operating systems. Includes libc reference.
URL: http://www.unix.org/single_unix_specification/

FASM
General-purpose assembler for both win32 and linux. Can include FASMLIB by sources, and produce executable directly without any linking.
URL: http://flatassembler.net

MASM
Win32-only commercial assembler with lot of license restrictions. Most popular assembler on win32 platform.
URL: http://www.microsoft.com/downloads/details.aspx?familyid=7A1C9DA0-0510-44A2-B042-7EF370530C64

NASM
General-purpose assembler for both win32 and linux. It's development has been resumed recently, but it is still inferior to YASM.
URL: http://sourceforge.net/projects/nasm/

YASM
This is NASM clone, with additional features (64 bit support etc.). Usage of FASMLIB in NASM and YASM is exactly same. Unlike NASM, this project is under development.
URL: http://www.tortall.net/projects/yasm/