Useful links: Technical editing, copying editing and working as a freelance
This list is not exhaustive and I take no responsibility for the content or accessibility of the sites.

Society for Editors and Proofreaders (SFEP)

Society of Indexers

Institute of Scientific and Technical Communicators (ISTC)

Society for Technical Communication (STC) – technical editors special interest group

Technical Editors' Eyrie (Jean Hollis Weber)

Purdue University Online Writing Lab

The Editorium

Free Pint

Wikipedia articles on technical writing, technical writer, editing, copy editing

American Psychological Association (APA) style guidelines

US Government Printing Office style manual

The Economist Style Guide

The Guardian Style Guide

British Library catalogue

University of Oxford Online library

US Library of Congress

European Union

Companies House

British Standards Online

ISO

US National Library of Medicine (Medline)

Science Direct

Ingenta Connect

Bartleby

Merriam-Webster OnLine

Acronym Finder