I originally heard about a device called an equatorial platform. This device rotates the scope essentially in harmony with the rotation of the planets around the earth (sidereal rate). It's quite ingenious, but pricey. The least expensive one I could find new was around $500-800!
There is also the option of building one yourself, if you have the woodworking and electronic skills necessary to do the job.
Since my hobbies are woodworking and electronics, I thought it was an ideal project to take on next! However, as fate would have it, I came across a used Tech 2000 Dob Driver II Rim system for sale for $100 on Astromart.com. The two motors used in the Dob Driver II are more expensive than $100. The red handheld unit where all of the "smarts" of the system are, is where all of the costs are located!
The Tech 2000 Dob Driver II system is also quite ingenious and much simpler in implementation. Instead of mimicking the rotation of the earth, it simply adjusts the alt-az mount to track the object you're observing. It contains two powerful stepper motors (one drives the rotation of the scope and the other drives the altitude of the scope) and with just a few seconds of training, will track an object without tedious polar alignment or telescope levelling which are critical for an equatorial platform.
New, their models sell for around $750 to around $1150! I was able to find a NOS (new old stock) Rim drive system for only $100 on Astromart.com. This is almost as good a deal as my Big Dobsonian was!
More photos of the Dob Driver II here:
I've got all the parts I need for upgrading the system, now all I need to do is to mount the drives and install the altitude bearings to have it work with my Coulter.
I've already notched out the baseboard for the azimuch motor:
Next, I need to cut out a circular area out of the rocker box so that I can add an 18" altitude bearing to the right side for the Rim drive motor.