Catching up on my magazine reading, I came across a great idea in the May issue of Better Homes and Gardens for making contemporary planters from concrete patio pavers like this one from Lowe's.
You just make a simple box using four square pavers as the sides, with no top or bottom. Glue the edges of the pavers together using a thin bead of landscape block adhesive, and then let the box sit for 24 hours.
Once the adhesive has cured, glue on another paver to form the bottom of the box, leaving some gaps in the bead of adhesive for drainage.
You can see pictures of the planters on Pinterest.