I liked the concept of @Kimframes spool but wanted to build my own parametric version so I could customize for wall-warts/chargers and make different sizes. I added a solid core to the center of the spool and gave it two slots so you can attach the spool to the power prongs of your wall-wart, which keeps everything tidy - both for storage and when in use.

To deploy the charger, pull the spool off the prongs and uncoil just as much cable as you need for the task. Then re-secure the extended cable through the cog teeth so the unused length stays neatly coiled and not tangled up in a mess on your desk.

I've posted the two most common variations I use as STL files: one for 3 mm cables and a slightly larger one for 5 mm cables. But if you want any other sizes, you can use OpenSCAD to configure your own.

Instructions

I usually print mine in PLA with 0.3mm layers and they work fine. 

No supports necessary. 

Assembly

Thread your cable through the center of the hollow half, then push the two halves together, trapping the cord in the center of the spool. Then just wind the cord around the spool and secure the end in the cog teeth.

You can even write the Voltage/Amperage of the charger on the face of the cog in large, friendly letters with a Sharpie. That makes it a lot easier to rummage through a box of chargers and quickly find the one you need.