Please feel free to add to this document
ArcGIS Metadata Toolkit: 10.5 (SDK for ArcMap)
Metadata in plain language (Peter Schweitzer)
Formal metadata: information and software (Peter Schweitzer) Note, the mp tool is shipped with ArcMap and accessible as mp.exe or using a GP tool.
NSDI Metadata Validation Service (web interface to mp, cns running on a server).
Metadata Wizard - A standalone FGDC CSDGM editor that works with ArcGIS 10.
FGDC Metadata Editor for ArcGIS 10 (this is the 9.x FGDC editor)