USA Native Plant Resource Masterlist
Because Google is totally useless and won't help you with ANYTHING
iNaturalist: Take photos of living things you see, post them, and the community will identify them for you. Data from iNaturalist is used in scientific research.
Wildflower.org Plant Database: Enter search criteria and find some plants. Very useful if you're looking for plants with specific qualities or know what you have in mind.
Native Plant Finder: This website is still in beta and is a work in progress, but it will show you plants for your area ranked by the number of butterflies that use them for their caterpillars.
WildflowerSearch: AMAZING resource for identification and for learning about new plants. Shows you where plants are native/not native, TONS of search filters.
Native Plant Trust: A New England organization, but probably useful to anyone.
Northern Forest Atlas: Great images and identification resources for trees; has good pictures of bark, seeds, buds, leaves.
FloraFinder: Another plant database site that's being slowly built up by a passionate nerd.
MonarchWatch milkweed by USA ecoregion: Tells you what milkweed species you should plant for monarch butterflies.
Native Beeology: Not plants, but a closely related subject.
I will add more and post an updated list as I find more.