The Agricultural Research Center (Dr. Duke's Phytochemical and Ethnobotanical Databases) is a valuable on-line tool for determining nutritional information of many food items, including numerous weeds.
If you select "Plant Searches -- Chemicals and activities in a particular plant", you can find out what vitamins, minerals, etc. are found in a given plant.
If you select "Chemical Searches -- Plants with a chosen chemical", entering a particular item like oxalic acid, vitamin C, calcium, etc. will produce a high/low list of plants with these chemicals. Remember that the information provided for any of these plants is an average. The range of values can vary considerably. Unless a particular plant has been tested, the precise amounts of any nutrients will not be know. These tools should only be considered a guideline. Farmers, for example, must send a precisely collected sample of any plants or hay out for testing if they want accurate nutritional data.
The USDA maintains a poisonous plant database. Their list is not complete, nor have they attempted to evaluate the validity, reliability, or scientific merits of the individual reports cited in the database.
The Weed Science Society of America maintains an extensive list of weeds, plus illustrations which may help you identify plants.
Nutritional info on grasses and hays: www.safergrass.org/
POISONOUS PLANT LISTSMelissa Kaplan has compiled a comprehensive list of edible and harmful plants with a description of their toxic effects.
See also the Cornell University Poisonous Plants Informational Database: www.ansci.cornell.edu/plants/