Cacti of San Diego County (San Diego Natural History Museum)
CalFlora - Over 20,000 photos of California plants (representing about 1/3 of the flora), over 790,000 observations of California plants and associated documentation.
Checklist of the Vascular Plants of San Diego County (SDNHM, SDSU)
Jepson Online Interchange for California Floristics - Search Specimen Data from the Consortium of California Herbaria or Search the Interchange using the Index to California Plant Names
Plants of San Diego County - Taxonomy of California Plants -- habitat and species photographs (Michael G. Simpson, SDSU)
U.S. Fish and Wildlife Service - Regularly updated information on threatened and endangered species, USFWS press releases, and a free e-newsletter.