Since trapezoid window measurements are unique, you'll need custom-made coverings. Although some designers can make curtains to your specifications, it's easier to use shades or blinds with trapezoid windows. Of the two, window shades are generally cheaper than blinds, depending upon the material.
Check in the Yellow Pages or conduct a Web search for companies that offer custom window coverings. Some mail-order companies may require you send in a template you've traced of your window; others may just have you make sure your measurements are accurate for all four sides, and that you specify the orientation (which side is the top, which side is on the right, etc). Local companies will be able to visit in person to take measurements.
Honeycomb cellular shades are an economical choice for trapezoid windows, as the pleats form air pockets that provide a layer of insulation and even protect against ultraviolet rays. These shades are made from one continuous piece of fabric without seams or rails. They can also be fitted for the angled part of trapezoid windows so that the bottoms can be raised at an angle and lie flat.
Woven wood shades, crafted from a combination of grasses, woods and bamboo, are only available for angle-top windows. The bottom of the shade is raised and lowered along the straight portion of the shade, and in the fully raised position, a triangular area at the top of the shade remains covered.
Horizontal wood blinds are stiffer than shades and therefore can only be raised or lowered in some trapezoid designs. In those cases where they can't be raised, you'll still be able to control light and privacy by tilting the vanes (slats) with a pull cord or rod.
Vertical blinds are controlled by using a special sloped track. With these blinds, the vanes are always stacked on the taller side of the window.
If all you want to do is diffuse the light in your room and you don't need to raise and lower the window treatments, you can use silhouette blinds. These blinds are made from a soft fabric that allows light through but doesn't allow anyone on the outside to see in. These blinds are stationary, with the vanes held permanently in the open position.