Below are a bunch of plant lists from random places I've been recently. They are by no means complete, and almost certainly have inaccuracies. (Remember that I have no formal training.)
Great Basin: Plants, birds.
Desert Southwest: Plants, birds.
Sierra Nevada: Flowers.
Great Smokies: Plants, birds.
Florida Keys: Trees.
New Zealand: Plants, birds.
Pasadena: Ornamental trees.
Supermarket: Cultivated foods.
Lists like these can be useful if you're having trouble making a positive identification of a plant in your guide books: look up the family or genus in my list and see what species are listed, then look for info on them on the web. (There are tons of pictures and botanical descriptions of just about every plant known to man on the web!) No guide book (except for tomes like the Jepson Manual!!) are complete. How many times have you found a plant that sort of looks like a couple shown in your guides, but doesn't exactly match any of them? How frustrating is that? Most of the time if you just patiently search for the genus on the web and start poking around you'll stumble on your plant. Plant lists like mine can help give you ideas what to search for.
I've tried to include only plants that I've positively identified, but occasionally I'll get a little overly optimistic. And some, like goldenods, I barely even tried to pin down the exact species! Generally these lists are compilations from several, even dozens in some cases, of sources, from guide books to National Park visitor center handouts to botanists I've met or gone walking with to websites with good pictures or descriptions. In some lists I've included approximate locations where I've observed species, although I'm usually pretty vague, as in "San Gabriel Mountains" or "Grand Canyon".
There are two particularly interesting lists. On the Pasadena list I've included the country of origin in most cases -- it turns out just about every tree in Pasadena is exotic! And my supermarket list contains some fascinating info about when, where, and by whom each plant was first domesticated. It really opened my eyes.