Taxonomic Tools
Keys, all taxa:
ZIMNES: by SAHFOS, Plymouth, UK. The goal of these pages is to provide a tool which can help both experienced zooplankton taxonomist and students just starting to learn identification of zooplankton. Currently, these pages are under construction but will eventually include a visual key. The taxonomic tree gives information about phyla, some images and references.
An Image-Based Key To The Zooplankton Of The Northeast (USA): by Jim Haney and students at University of New Hampshire, USA. Covers common rotifers, cladocera, copepoda, and some other arthropoda found in New Hampshire coastal waters.
Crustacean.net: by the Australian Museum. Includes amphipoda, anaspidacea, anomura, calanoid copepods (families), isopoda, mictacea, mysidaea (families, subfamilies, tribes), remipedia, spelaeogriphacea, stomatopoda, tanaidacea. A few families have interactive keys which require Intkey to be installed on your computer (free download).
Guide to the Marine Zooplankton of south eastern Australia: Both an image based key and a diagnostic tool to distinguish local groups. Eventually each links through to separate species/taxon pages. Diagnostic key requires free download of Lucid software and java-enabled web browser.
Keys, specific taxa:
Ciliates:
"The user-friendly guide to coastal planktonic ciliates" by Planktonic Ciliate Project, University of Liverpool, UK. The guide is quite nice and the experts were involved, although few species are listed.
Copopods:
* Diversity and Geographic Distribution of Marine Planktonic Copepods by Claude Razouls, Francis de Bovée and Juliana Kouwenberg at Observatoire Océanologique de Banyuls, Université Pierre et Marie Curie (Paris) - thorough, continually updated, excellent resource. Includes extensive references, figures, maps of all described species of copepods. (rate this site)
"A Key for the Identification of the More Common Planktonic Copepoda of Indian Coastal Waters" by L. R. Kasturirangan, 1963
Chaetognaths:
Chaetognatha of the World: by Annelies Pierrot-Bults, Zoological Museum, University of Amsterdam and the World Biodiversity Database. Covers all described species with an image, description, references, etc. for each.
Other types of References:
pdf's:
A collection of
classic taxonomic texts: free downloads: copepods, 
medusae, 
ctenophores
More classic copepod texts from the Internet Archive: Brady, Breeman, Giesbrecht, Sars, etc.
Plankton of the offshore waters of the Gulf of Maine. By Henry B. Bigelow. 1926: pdf; NW Atlantic; somewhat out of date. Available on the Biodiversity Heritage Library website.
websites:
The World of Copepods: Bibliography, taxonomic list, databases of researchers, specimens, genera. Also, links to other copepod sites.
CD's/DVD's:
The DVD version of the book "Zooplankton of the South Atlantic Ocean" has a guide to many taxa. Available from ETI Bioinformatics, Multimedia Interactive Software.
Also from From the same source find Copepoda - Aetideidae of the World Ocean.