Line Out Docks (LOD) are typical 30-in connectors which snap into the bottom of an iPod or iDevice to grab the line-level audio information without a hint of the headphone amplifier. They can be big or small, and all speaker docks make use of LODs in one form or another.

A few good companies are:

ALO - Audio Line Out who make the excellent ALO Rx headphone amp

Twisted Cables (no website yet) - Brainchild of Headfier Qusp (Jeremey Glover - Email), fastidious and new cable maker

Qables - From the Netherlands, home of the excellent portable headphone amp, iCube