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

Editorial Freelancers Association (US)

Editors Association of Canada

Technical Editors Eyrie Newsletter (Jean Hollis Weber)

Purdue University Online Writing Lab

The Editorium

Free Pint

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

University of Colorado at Boulder Style Guide

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

Office of Public Sector Information (OPSI) (including UK legislation)

European Union

Companies House

British Standards Online

ISO

Bureau International des Poids et Mesures

US National Library of Medicine (Medline)

Index Medicus

Science Direct

Ingenta Connect

Dictionary of Cell & Molecular Biology

Bartleby

Merriam-Webster OnLine

Amazon

Blackwell Online Bookshop

Acronym Finder

Google