Also the name of the magazine published by the Role Playing Gamers Associaton. Although owned by Wizards of the Coast, it is not fully evil, instead providing news, information, and entertaining gaming content to members of the RPGA Network.
The magazine subscription is one of the benefits of membership in the RPGA, and it tends to come every other month. Polyhedron contains new monsters, intriguing NPCs (usually with interesting plot points too), magical items, information or gossip about the game worlds, and letters and editorials.
Compared to Dragon or Dungeon (the two main D&D magazines on the market), Polyhedron pales in comparison. It's main purpose is to inform and serve the RPGA's gamers and to supplement the game worlds of the RPGA (such as the vaious Living campaigns). I probably wouldn't pay for it at a stand or for subscription, but it is nice to read every so often and can have fun new items to spice up your gaming.