Bookcase headboards are not new, but you may not know that you can use real bookcases to do the job. If you have one that is as wide as or wider than your mattress, push it behind your bed. Anchor it firmly to the wall and fill every shelf that isn't covered by bedding. If you have two taller bookcases, stand one on each side of the bed in lieu of nightstands. Bridge the tops with a board painted to match; the board will not only be an extra shelf, it will complete the "frame" around the head of the bed. Hang a curtain, a special picture or a mirror on the wall in between to complete the look.
Free-floating shelves work well as headboards; you can center one over the bed or cover the wall with them floor to ceiling to showcase collections, hold books or store personal items. You can buy coordinated shelving "systems" that will look like built-ins or hang an eclectic collection of shelves, cabinets and display cases that you've collected at thrift stores and yard sales. Leave them as-is for a rustic look, or paint them all the same color for a unified display.
Storage cubes are infinitely stackable and come in colors to suit every décor and prices to suit every budget. Buy an assortment of cubes made from wire, plastic, or wood---cardboard or crushable fabric are not sturdy enough to use as a headboard. Get cubes that "lock" together---or fasten them yourself using hot glue or wood fasteners---and experiment with stacking them until you get a configuration that pleases you. Then place the stack between the bed and the wall.
If you have oak filing cabinets, place two of them back-to-back with their sides against a wall and butt the head of the bed up against the other sides. This gives you a rich, paneled wooden "headboard" with drawers opening out on both sides and a wide top to hold books and lamps. Use matching metal filing cabinets for a more "steampunk" look; this works especially well in a teen's room. Be sure to choose filing cabinets that are deep enough to provide the necessary width with placed back-to-back.