mubench is an in-depth, low-level benchmark for x86 processors. Its primary goal is to provide useful information for people who optimize assembly code and for people who write compilers. It measures latency and throughput for each individual instruction (sometimes several forms of the same instruction), as well as the throughput of arbitrary instruction mixes. The results produced by mubench are typically an order of magnitude more detailed than those found in AMD or Intel manuals.

Vx32 is a user-mode library that can be linked into arbitrary applications that wish to create secure, isolated execution environments in which to run untrusted extensions or plug-ins implemented as native x86 code.

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

AMD's new Barcelona processor is using a really strange cache hierarchy

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

Technical docs on Athlon and Opteron