I have a number of plants that are solely placed because of their beautiful and interesting foliage. Caladiums are among the brightest and best plants for shade precisely because of their delectable foliage. The caladium above is called White Queen. It makes a very bold statement in a shady area.
Here's another picture with more caladiums. Be sure to put these in a shady, moist area. They should be put out once the weather turns hot. In the very deep South, they often return after over-wintering in the ground, but elsewhere you must dig them up in the fall.
Another bold statement in a tropicalesque bed is this lush Bengal Tiger canna. In contrast to caladiums, these do best in bright sun. They will still do fine in part shade though. The leaves are grean with yellow stripes and each leaf has a red highlight all around the edge. These are bold and beautiful and will spread rapidly to make many more. Keep them well fertilized and watered. In my zone 8 area, these never need to be dug up and will come back every year.
Yet another plant that makes for striking foliage is the ornamental sweet potato. This particular one is a chartreuse-green variety called Sweet Caroline. You can also get these with burgundy foliage and it looks just as good. These will easily fill out an area under a tree in a dry, sunny, poor-soiled place. They have almost no pests and require no care once established. They will completely die once a frost hits and must be treated as an annual.
These plants are just some of the great ones for filling out an area between flowering perennials and shrubs. They give a garden that "full" look that is so desirable. Don't neglect the fillers!