Before I get too far in this instructable, I would like to say that this was not my original idea. You can see two implementations of this idea already on Flickr. The links are:
http://www.flickr.com/photos/mdweezer/322631504/in/set-72157594420700670/
http://www.flickr.com/photos/samwibatt/1610784412/

On with the instructable! This documents how to turn a regular disposable camera into a high voltage power supply capable of driving 2 or 3 medium-sized nixie tubes, for roughly $8.

***Disclaimers***
This instructable works with voltages in excess of 250V. This is more than enough to give you a potentially fatal electric shock if handled incorrectly. If you are unfamiliar with how to work with high voltage, please refrain from performing this instructable. Exercise caution throughout the following steps to avoid electrical dangers. If you choose to undertake this instructable, you do so at your own risk.

This instructable involves soldering. A soldering iron becomes very hot during its use, to the point where it can cause instant second-degree burns. Exercise caution throughout the following steps to avoid burns. If you choose to undertake this instructable, you do so at your own risk.

Step 1: Gather Materials and Tools

For this instructable, you will need:

A disposable camera
A potentiometer of 100Kohms or higher
A resistor of 50Kohms or higher
Wire cutters
A small screwdriver (may not be needed, depends on your camera)
A multimeter
A soldering iron
Solder
Red wire
Black wire

Step 2: Disassemble Camera

This step is going to vary depending on the camera you choose to take apart. I will post a pictorial description of how I took mine apart, but know that not all cameras are alike.

Also, this step is probably one of the more dangerous steps, because you do not know if the capacitor is charged currently. Do not touch the capacitor or the flash circuit at this point, it may still have energy stored that could electrocute you. Being electrocuted is a bad thing.

Once you expose the capacitor, I highly recommend discharging it. This can be done by touching both wires coming out of the bottom of the capacitor with a screwdriver or other metal object. Make sure the object you use to discharge with has an insulated handle, and only use one hand to minimize risk of electricity flowing across your chest. I also recommend wearing safety goggles, because if the capacitor is charged, sparks will fly, and you don't want one of those sparks in your eye. (sparks can also known as superheated airborne metal fragments)

Step 3: Attach Wires.

To make this circuit more convenient to use, it is time to add some wires. Notice the capacitor has a stripe on it. The black wire will connect to the lead beneath the stripe, and the red wire will connect to the other lead.

Step 4: (optional) Make It Safer

This step is optional, but I highly recommend it.

Solder a resistor across the leads of the capacitor. This will enable it to self-discharge in a controlled manner. You can use any resistance over 50Kohms, but the lower the resistance, the faster the circuit will decrease to a safe voltage. I clocked mine, at 100Kohms, the circuit went from full power (about 230V) to a more reasonable 20V in about a minute. 50Kohms makes the circuit safe in about 30 seconds.

Alternatively, you can completely remove the capacitor and never need to worry about stored charge again.

Step 5: Case It Back Up.

In the interest of maximizing safety, I chose to put the circuit back into the camera's shell. I ran the wires out of the rear viewfinder window, so I had to poke a little piece of plastic out of the window.

Step 6: Create the Circuit and Use.

Now, wire up the circuit for the nixie tube. Put the potentiometer in between one of the leads and the nixie tube, and adjust as needed. With my IN-12A tubes, 200Kohms pass about .3 mA, which is enough to illuminate the tubes, but is not very bright. Experiment with your own tubes (use the multimeter here). Sorry for not showing pictures of the calibration process, but this circuit blew out my multimeter when I accidentally created a short circuit. Like I said, be careful.

When the camera is on, the high voltage is present, so please exercise caution when using this.