Agave americana
This is the most commonly encountered agave. It gets quite big, 1.5m -2m across on mature plants, grows fairly quickly for an agave and offsets like crazy. Larger plants are quite cold tolerant, and once the size of a dustbin should be able to remain outside. In the mild wet conditions of the west country they seem to do well, in the drier colder parts of the country I don't know, but I think anywhere they are planted out they need good air circulation. I have heard of people in the north of the country growing them to a good size when planted out by covering with plastic to keep the rain off. Eventually they will flower, but I have only seen one flowering in the Scilly Isles. Plants are variable from different sources - some are quite glaucous and silvery, some have very broad leaves - there are distinct sub-species in the wild - so shop around and you will get some nice variations. There was a particularly nice silvery form distributed a few years ago in the trade as Agave franzosinii - it wasn't A. franzosinii but a nice plant nevertheless. None of the following variegated forms are as hardy as the plain species. ( 1999 )
I know this plant as having irregular but distinct bands of creamy-yellow that aren't restricted to the leaf margins, and the leaves are narrow and wavy.
This is the one I grow - it has yellow variegation that shows mainly on the edges of the leaves, which are themselves broad and slightly recurved
I have only just managed to get hold of this - it is rarely seen in cultivation. The colours are the reverse of 'Marginata' in that the leaves have a yellow central band.
This is similar to the above except that the central band is pale cream to white. It remains quite compact and is often difficult to track down.
This one has very thin streaks of yellow distributed throughout the leaves, not just confined to the leaf margins or a central band. Again, rarely seen - even more rarely offered for sale.