Core Text for macOS

Uniscribe (with Universal Shaping Engine) and DirectWrite for Microsoft Windows

HarfBuzz, a cross-platform library

Pango, a cross-platform library which nowadays incorporates HarfBuzz

Writing systems which require complex text layout:

Most of the Brahmic family of scripts

Tengwar (diacritics and numbers)

image complex text layout

define complex text layout

wikipedia complex text layout

weblio complex text layout

kotobank complex text layout

jisho complex text layout