Gunnera tinctoria and tree ferns in Golden Gate Park. (Older synonyms are G. chilensis and G. scabra).

Charles Darwin, when exploring the island of Tanquil (part of modern Chile) in 1834, noted the "noble" Gunnera.

"I one day noticed, growing on the sandstone cliffs, some very fine plants of the panke (Gunnera scabra), which somewhat resembles the rhubarb on a gigantic scale.
The inhabitants eat the stalks, which are subacid, and tan leather with the roots, and prepare a black dye from them. The leaf is nearly circular, but deeply indented on its margin. I measured one which was nearly eight feet in diameter, and therefore no less than twenty-four in circumference! The stalk is rather more than a yard
high, and each plant sends out four or five of these enormous leaves, presenting together a very noble appearance." Voyage of the Beagle, Chap. XIII

This is the subgenus of the distinctive and enormous "Chilean rhubarb" species of the tropical South American subgenus Panka known from parks and botanical gardens through Europe and America. The largest leaves are reported from Gunnera magnifica in the Colombian rainforest with leaves routinely over 2 meters in diamter. Similarly large leaves are reported from the Brazilian Gunnera manicata, which is found in many temperate gardens. Amongst the old garden populations of this species are those from the Lost Gardens of Heligan:

 These are also sometimes called "poor-man's parasols" because the large leaves can be used as umbrellas, especially those of less deeply lobed species such as G. insignis (syn. G. killipania), featured recently on the cover of The Botanical Review. This photo was taken in the Berkeley Botanic Garden in 1994/95. The photo below shows an offshoot of this plant in April 2007, with smaller early spring leaves and flowers.

For an excellent photo of G. insignis in its natural habitat in Costa Rica, see this, from Wikipedia commons.

Some species in this genus are now regarded as invasive plants in mild temperate regions, and naturalized populations are reported from New Zealand, Australia, Ireland, Cornwall and parts of coastal California around the San Francisco Bay Area.

Nevertheless they remain a popular and impressive garden plant, especially the similar G. manicata (larger leaves) and G. tinctoria (smaller leaves and more succulent inflorescences), see e.g. here, or here, or here.

For the impressive Gunnera petaloideaof Hawaii see thee photos from Dr. G. Carr, University of Hawaii. A range of other Panke photos can be found here.