Metasm is a cross-architecture assembler, disassembler, compiler, linker and debugger.

Sassy is a portable assembler for 32-bit or "protected-mode" programming on the x86 architecture. Sassy is based on Henry Baker's COMFY-65 compiler, which he wrote in PDP-10 Maclisp, subsequently ported to GNU Emacs Elisp, and which targeted the MOS 6502

A list organized by what it does rather than alphabetically by mnemonic

Latency and throughput measurements for a set of common integer and SIMD operations on P4, Athlon, and Opteron machines. Very helpful for optimization purposes.

The GCC docs are pretty craptacular, so this was helpful

Description of the core PowerPC instruction set

IBM PowerPC assembly documentation