Many pieces of furniture can be converted into interesting and functional planters. In particular, chairs featuring wicker seats are ideal. However, anything with a seat that's relatively easy to cut a hole through will do. First, decorate the chair however you'd like. Then make a hole in the seat so that a standard a potted plant will slide into it without falling through.
If you're converting a small dresser or similar item, first weatherproof it with a sealant. Next, cut holes to allow air flow to the drawers below. Then, drill a few holes in the bottom of each drawer for drainage and fill them with soil. Be sure to stagger the drawers to display your plants to their fullest potential. If the dresser is too heavy or feels like it might tip forward, secure it to an exterior wall or stake it to a tree or the ground.