Power Strip liberator: This cord plus into your power strip and provides a 1’ extension cord to facilitate plugging in wall wart plugs. It allows you to plug in large plugs without using adjacent outlets. Also, it has a pass through plug.
Squids: This type of cord looks like an octopus – hence the name. The unit plugs in and then separate cords branch out each ending in an outlet. http://www.powersquid.com/
Two-fer style connectors: like the lighting plugs – one cord branches off to provide two outlets. Available at: http://www.buyextensioncord.com/ Also available (from lee valley) are cords that allow 3 instead of 2. The squids mentioned above allow for 5.
Multi-Outet cords. These cords have staggered outlets along the length of the cord. You can check them out at: http://www.leevalley.com/gifts/page.aspx?c=2&p=51162&cat=4,104,53212&ap=2.
They are convenient to string around a shop or location where not enough outlets exsists. A great use for this is back stage for clip lights.