The adder-substracter is a nifty little device that can 
add or substract the values of 2 inputs on 2 outputs.
There are many applications for this design, probably 
the most well know of which is for precision steering in 
robots or threaded vehicles(tank drive)

The motor connected to one of the inputs lets you drive 
forward or backwards in a straight line, the other input 
regulates the steering.

The yellow design is by Pheonix 
(http://www.brickshelf.com/cgi-bin/gallery.cgi?f=25110) 
but I have updated the ldr files to correct some minor errors.

His design in turn was based on one by Denis Cousineau. 
On his website you can get a complete explanation of what 
an adder-substracter is and how it works:

http://mapageweb.umontreal.ca/cousined/lego/1-Varia/Adder/adder.html

The design with the red and green input ports is by Leo
(http://staff.science.uva.nl/~leo/lego/diff.html)

It's rather bulky and can't be used 'as is' (you need e few more 
gears to use it in a vehicle) but it probably was the first 
adder-substractor made of lego, documented on a webpage.

The third design is an adaptation of another existing design, 
but I can't trace the owner. (If you know who made it drop me 
a line and I'll credit him)

linmix@gmail.com