Botany
Plants of the World Online
http://powo.science.kew.org/
USDA Plants Database
https://plants.usda.gov/java/
World Flora Online
http://www.worldfloraonline.org/
Botanical art and artists
https://www.botanicalartandartists.com/
A Grammatical Dictionary of Botanical Latin
https://www.mobot.org/mobot/latindict/search.aspx
Chemistry
ChemSpider
http://www.chemspider.com/
ChemSpider (BETA)
https://beta.chemspider.com/
Spectral Database for Organic Compounds (SDBS)
http://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi
Complementary and Alternative Medicine (CAM)
https://www.cancer.gov/publications/pdq/information-summaries/cam
Ethnopharmacology
The Journal of Ethnopharmacology
https://www.journals.elsevier.com/journal-of-ethnopharmacology
Herbal Medicines
NHS Herbal Medicines
https://www.nhs.uk/conditions/herbal-medicines/#potential-issues-with-herbal-medicines
National Center for Complementary and Integrative Health (NCCIH)
https://www.nccih.nih.gov/health/herbsataglance
Historical Plant Use 
Botanical.com
https://botanical.com/
Herbs in History
https://www.ahpa.org/herbs_in_history
Natural Products
Natural Products online (UK)
https://www.naturalproductsonline.co.uk/
Pharmaceutical Drugs
Medline Plus
https://medlineplus.gov/druginfo/herb_All.html
drugs.com
https://www.drugs.com/drug_information.html
Plant Names
The International Plant Names Index (IPNI)
http://www.ipni.org
Medicinal Plant Names Services (MPNS)
http://mpns.kew.org/
Regulators
US Food & Drug Administration
https://www.fda.gov/
UK Medicines and Healthcare products Regulatory Agency (MHRA)
https://www.gov.uk/topic/medicines-medical-devices-blood/herbal-homeopathic-medicines