Names and Nomenclature
International Plant Names Index (IPNI), includes Index Kewensis, Gray Card Index, etc.
Index Nominum Genericorum.
Index Fungorum.
W3 TROPICOS has a great deal of nomenclature and specimen information.
International Code of Botanical Nomenclature, Vienna Code in English, Tokyo & St Louis Codes in several languages.
Kew Author Abbreviations, included within the IPNI site (see also the HARVARD version).
Botanico-Periodicum-Huntianum (BPH & BPH/S) standard journal abbreviations (Harvard version).
Taxonomic Literature 2(TL2) standard book abbreviations (scanned pages of the original book, see also the HARVARD database version).
Linnaean Plant Name Typification Projects includes links on the homepage to historic online herbaria at the NHM.

Kew Record of Taxonomic Literature.
Gabriel: Gateway to European National Libraries.
RBGE Library.
Natural History Museum, London, Library.
RBG Kew Library. access to historical publications:
Biodiversity Heritage Library, with digitized literature from 10 major botanical libraries (including Missouri Botanic Garden's Botanicus
Google Books, with a surprising depth of digitized literature - maybe in restricted form,
Gallica: French Libraries Early literature, good for online access to early botanical works, e.g. Candolle, Durande, Jussieu, Linne (Linnaeus), etc.
Botanical E-Journals.
Curtis' Botanical Magazine.
...and to help understand what is written:
Flora of Australia Glossary.
Botanical Latin Translator.
Yahoo Babelfish Translator.

Herbarium Specimens
Index Herbariorum.
Many Herbaria are starting to put their collections online, either just text label data or with images of the specimens. Global Biodiversity Information Facility, GBIF, is a portal to institutes with online collections linked to GBIF, and good starting point.
Other portal sites include Fairchild Virtual Herbarium Web Portal and the Worldwide List of Internet Accessibe Hebaria, but the latter is rather out of date now.
Major herbaria will probably have their own online herbarium so check their institute website (e.g. RBGE's herbarium catalogue).
National Biodiversity Network, NBN, facility to for UK biological records, mapping, etc.
The Linnean Herbarium is now online. work out who collected them and where they were collected:
Harvard University Herbaria Collectors Index.
Worldwide Directory of Cities and Towns and US National Geospatial Intelligence Agency (formerly NIMA) Public Geographic Names Server,
NGA GEOnet Names Server is another resource for putting a place to a name,
MapQuest, for checking positions, deciding what political boundary it is in,
BioGeomancer, a facility to help geo-locate old biological collections,
East Himalayan Gazetteer of Plant Collecting Localities
Flora of Nepal Botanical Locator
...and to plot them on a map:
Google Earth, a download programe that is just brilliant for a variety of geo mapping tasks,
GPS Babel, a download utility for converting GPS data between formats (including Google Earth),
DIVA-GIS a free mapping programme good for distribution maps, routes, etc.

Living Collections
As for online herbaria, there are several Botanic Gardens with online catalogues, e.g. RBGE Living Collections.
Those usings BG-BASE and HTMLEXPORT are linked through the RBGE Multisite Search.

General Information
Any major searchengine, e.g. Google, Altavista, Yahoo.
Internet Directory for Botany, an alphabetic list of sites with a search facility.
Wikipedia, often has informative pages on families, genera and species,
Encylopedia of Life
Catalogue of Life
RBGKew Electronic Plant Information Centre, (EPIC).
Flowering Plant Gateway, includes synopses of major classifications, e.g. Cronquist, Takhtajan, Thorne, APG.
Angiosperm Phylogeny Website (APG).
IOPI Global Plant Checklist, provisional.
Species Plantarum Project.
There are lots of sites serving images, e.g. Texas A & M University, or use a search engine (e.g. Google Images) but beware of misidentifications!

Taxonomic group-specific websites are growing in number, e.g. Umbellifers, Zingiberaceae, Sapotaceae. Try typing in the taxonomic group name into an internet search engine.
International Association of Plant Taxonomy.
TAXACOM general systematists listserver (plants & animals)
Mycology Net
Global Taxonomy Initiative, GTI.
Taxonomic Databases Working Group.
Herbarium Supply Company, very good for fieldwork collecting equipment.

Online Floras
There are an ever-increasing number of online Floras, the following offers a selection of those available:
Flora of China Checklist.
Myanmar Checklist.
Flora of China.
Flora of North America.
Flora of the Canadian Archipelago, includes DELTA descriptions and keys.
Flora of Japan.
Flora of Europe photgraphic guide.
Flora Himalaya Database, from Savoie University.
Flora of Bhutan Umbelliferae, prototype.
Flora of Nepal.

Online Keys and Description Coding
ActKey, major groups in Flora of China, plus several others, e.g. Hansen & Rahn's classic key to Angiosperms.
Hansen & Rahn, an online version of the classic punched card Key to Angiosperms.
DELTA and INTKEY homepage.
LUCID homepage.

eTaxonomy and Cybertaxonomy
Recent developments have seen the internet used for collaborative taxonomic research. See:
EDIT, European Distributed Institute of Taxonomy.
Scratchpads, biodiversity online

Mark Watson, January 2010