University of Maryland Extension (UME).  Provides unbiased research-based educational information for residents across Maryland. 
University of Maryland Extension (UME) in Spanish.

Specific UME sites of interest

Home and Garden Information Center (HGIC) provides access to a wide variety of subjects relevant to home gardeners.
Ask Extension: Submit questions or photos to UME for problem diagnosis or plant ID.
Soil Testing. Why and how to have your soil tested, and where to send your samples. Order testing kits from the University of Delaware Soil Testing Program.
Invasive Plants. Alternatives to and control of invasive plants in the home landscape.
Montgomery County Master Gardeners. Trained volunteers of UME; information on what they do and how to become a Master Gardener.

General interest

Missouri Botanical Garden Plant Finder. Search over 7500 plants by scientific name, common name, or plant characteristics. Includes photos, descriptions, requirements, problems.
Deer-Resistant Plants. List of deer-resistant plants compiled by Mt. Cuba Center and the Pennsylvania Wildlife Preserve.
Sustainable Landscapes. University of Delaware Botanic Gardens provides links to information on sustainable landscapes practices.

Native plants

Audubon Native Plants Database. Find plants native to your zip code.
ChooseNatives.Org. A list of native plant nurseries in the Mid-Atlantic region, resources for invasive plant removal and more.
Mt. Cuba Center. A public garden and botanic center located in Delaware, devoted entirely to native plants. Their website provides a native plant finder and the results of their native plant trials.
Wild Ones. An organization that advocates for striking home landscapes to benefit wildlife and people, with downloadable garden designs for ecoregions throughout the country. 

Pesticides

Organic lawn care from Montgomery County. Also information on the county pesticide law.
Pesticide Information from the Maryland Dept. of Agriculture.
Mosquito Control.  Bite prevention, repellents and control methods from the U.S. Environmental Protection.

Criteria for inclusion: trusted sources such as universities, government agencies, or non-profit organizations. No attempt is made to be comprehensive.