Some useful links

Herbarium/specimen resources:
Index Herbariorum: http://sweetgum.nybg.org/science/ih/ (index to world herbaria with basic information)
SERNEC: http://sernecportal.org/portal/ (consortium of herbaria for the southeastern U.S.)
iDigBio: https://www.idigbio.org/ (occurrence data aggregator and digitization hub for the U.S.)
GBIF: https://www.gbif.org/ (occurrence data aggregator)
IPNI: https://www.ipni.org/ (nomenclatural resource)
Tropicos: http://www.tropicos.org/ (another nomenclatural resource, and an access point to MO specimens)
The Plant List (TPL): http://www.theplantlist.org/ (nomenclatural and taxonomic resource)
POWO: https://powo.science.kew.org/ (successor to TPL with geographic, taxonomic, and other information)
S-NET: http://science-net.kahaku.go.jp/ (species occurrences in Japan)
Chinese Virtual Herbarium: http://www.cvh.ac.cn/en (species occurrences in China)

Environmental Data:
SoilGrids: https://soilgrids.org/ (global interpolated soil data)
Global 1-km consensus landcover: https://www.earthenv.org/landcover (global landcover remote sensing data)
Global 1,5,10,100-km Topography: http://www.earthenv.org/topography (global data on various topographic indices)
WorldClim: https://www.worldclim.org/ (global climatic data, including the much-used BioClim variables)
PaleoClim: http://paleoclim.org/ (global paleoclimatic data for a subset of BioClim variables back to the Pliocene)
MARIS: https://maris.mississippi.edu/​ (cultural, geoscientific, and other information for the state of Mississippi)

eFloras:
http://panarcticflora.org/ (Panarctic Flora)
https://www.nzflora.info/ (nzflora)
http://legacy.tropicos.org/Project/FN (Flora de Nicaragua)

Taxon resources:
Legume Data Portal: https://www.legumedata.org/
Global Compositae Database: https://compositae.org/aphia.php
Astragalus of the World: https://astragalusofworld.com/
Classification of Symphyotrichum​: https://uwaterloo.ca/astereae-lab/research/asters/symphyotrichum/classification-symphyotrichum
Taxonomy of rhizobia and agrobacteria: ​https://sites.google.com/view/taxonomyagrorhizo/home?authuser=0
Heuchera Research: https://sites.google.com/site/ryanheuchera/main-page