Other Useful Resources
Both Tropicos and the International Plant Names Index provide invaluable nomenclatural details on plant names; Index Fungorum has a similar function for fungi (including lichens). Tropicos also provides an online search of the Missouri Botanical Garden’s herbarium holdings, and many other goodies.
The Encyclopedia of Life (EOL) and the Tree of Life Web (TOL) aim to provide a webpage for every species on Earth, including, of course, all the plants. In general, EOL is more focused on species, and TOL on more inclusive taxa (genera, families, other clades). Currently, TOL is much more detailed where it has coverage (might we humbly suggest perusing the leptosporangiate ferns, or the cheilanthoids or the Ruelliaea), but that coverage is spotty.
Plantsystematics.org is a rich site, especially for photographs (it also hosts a useful georeferencing tool).
The angiosperm phylogeny website is a massive compendium of information on the world’s flowering plants and their evolutionary relationships to each other. Frequently updated, it is one on of the few places where one can hope to find a synthesis of the most recent data.
Natureserve Explorer is a very useful resource for information on distribution, ecology, and rarity for North American plants (and animals, too, for that matter). Among its most useful features is a list of the global (G), national (N), and state/provincial/territorial (S) rarity ranks for all recognized taxa, from both Canada and the USA.
The Global Biodiversity Information Facility (GBIF) provides access to over 200 million biodiversity records, most of them georeferenced. Their exporting and mapping tools make this site invaluable for those looking at broad scale ecological and diversity patterns.
Index Herbariorum provides contact information and collection summaries for the world’s herbaria.