These libraries fall into two categories: (1) C/C++ libraries that makes use of SIMD or other specialized instructions, (2) Node.js libraries that provide fast implementations one way or another (SIMD or just careful optimization).

Submissions welcome!

• base64 - Base64 encoding/decoding.
  Type: C99 library.
  Node.js bindings: https://github.com/lovell/64 ISA: SSSE3, SSE4.1, SSE4.2, AVX, AVX2, NEON; OpenMP.

• bswap - Byte-swapping faster than Node.js's built-in Buffer.swap16/32/64().
  Type: Node.js module.
  ISA: SSSE3, AVX2, AVX512, NEON.

• fast-hex - Hex encoding/decoding.
  Type: C++ header and Node.js module.
  ISA: AVX2.

• ISA-L - crc, erasure coding and deflate in the Intel Intelligent Storage Acceleration Library (ISA-L)].
  Type: C and assembly.
  Node.js bindings: https://github.com/primitybio/node-isal
  ISA: Runtime dispatch using up to AVX-512

• levenshtein-sse - Levenshtein distance.
  Type: C++ header.
  ISA: SSSE3, SSE4.1, AVX2

• libjpeg-turbo - JPEG library.
  Type: C library.
  ISA: MMX, SSE2, AVX2, NEON, AltiVec.

• Ivan Dimokvic's implementation of MWC1616 pseudo-random number generator. His original code post is no longer online, but a version of it is here: https://github.com/andeplane/MWC1616-PRNG-CPP.
  Type: Originally C, that repo is C++.
  ISA: SSE or SSE4. (In my benchmarks on a Skylake processor, the SSE implementation is faster than the SSE4 implementation.)

• sse-popcount - A few dozen implementations of population counting.
  Type: C++ implementations with benchmarks.
  ISA: SSE, SSSE3, AVX2, AVX512 or NEON

• sse4_crc32 and its wrapper with JS-fallback fast-crc32c - CRC32C calculation for buffers.
  Note: ISA-L (above) is up to ~3.4x faster.
  Type: Node.js module.
  ISA: SSE4.2.

• turbo-net - Low-level TCP library faster than Node.js's core TCP module.
  Type: Node.js module.
  ISA: Generic.

• volk - Librarary of vector-optimized kernels. A lot of these look like they will be memory throughput-limited, but they show how to do various tasks if you want to combine them to make your own fast functions.
  Type: C.
  ISA: Various (looks like up to AVX2).