LSynth is an LDraw System Lego CAD tool. It allows generation of flexible items - like hoses, cables, rubber bands, tank treads and similar. It has (slightly buggy) object avoidance, and with positioning of parts using standard software like MLCad is quite easy to work with.

It is currently the only tool of its kind for Lego that I am aware of - and can be very handy. Take a look at my Pneumatic Engine for an example. All the hoses in the image have been generated with LSynth.

Pneumatic Engine

The tool is available from the LSynth Website as well as the LDraw System foundation.

You may note that he has an example pneumatic circuit which behaves in a similar way to mine.

(paid links)

Lego Software Power Tools Including Ldraw, Mlcad and Lpub There is a large plethora of Lego related software. This book helps you navigate, choose and use this software effectively. As it was written by the author of LSynth - Kevin Clague and two other Lego fanatics - there is no reason to doubt its usefulness.
LSynth The home of LSynth - you can download it and get more info there.
LDraw Foundation This organisation are the custodians of Lego related tools and software - and an essential hub for the Lego CAD community.
Lugnet Lugnet is the primary discussion area for all things Lego, and have a number of forums for discussing Lego CAD.