Produce your own single-sided flexible printed circuits using a solid ink printer, copper-coated  polyimide film, and common circuit board etching chemicals.

You will find flex PCBs inside most cellphones or similar miniaturized gadgets. Flex PCBs are useful for making tiny cables and extremely lightweight circuits.

Step 1: Get Copper-coated Film

Get some thin sheets of polyimide which have copper on one or both sides. Polyimide is a yellow polymer with a high melting temperature and is sometimes called Kapton. A common type of copper-coated polyimide is DuPont "Pyralux" material.
Pyralux sheets come in many different varieties of polyimide thickness, copper thickness and adhesive thickness (the "adhesive" is between the copper and polyimide holding everything together.)  Copper thickness is given in oz per square foot, while adhesive and Kapton thickness is given in mil (1 mil =0.001 inch).

Pyralux LF7062 (pictured) has 1/2 oz Cu, 1/2 mil adhesive and 1 mil Kapton. This works OK but is a bit thin and crinkly for the printer to handle.
LF9120 has 1 oz Cu, 1 mil adhesive and 2 mil Kapton - seems to work best in the printer
LF9210 has 2 oz Cu, 1 mil adhesive and 1 mil Kapton - stiffer, but works OK

Other options are double sided copper ( a sandwich of Cu/Kapton/Cu held together with adhesive) and a roughened surface, denoted by R at the end of the part number.

The roughened sheets and double sided sheet work OK. However, Pyralux with 2 oz or thicker copper can be difficult to feed to the printer, especially if there is copper on both sides.

Cut the Pyralux sheets to 8.5x11 or 8.5x14 inches with scissors or a knife. Avoid smudging the copper with fingerprints or oil, which can block the etch solution later. To protect the printer, try to keep the edges relatively flat and free of burrs.

Step 2: Use a Solid-ink Printer

For direct printing on the copper film, locate a solid-ink printer. These are commonly confused with laser printers, but instead print melted wax. Unlike most inkjets, the wax makes a good protective layer for copper etching, and unlike laser printers, solid ink printers don't rely on locally charging the paper surface, which could be troublesome when the paper is replaced by a copper sheet.

Some models are Tektronix Phaser 840, 850, 860, and Xerox Phaser 8200, 8400, 8500, 8560, and 8860. You might find one in an office. Most Phaser models are regular laser printers, so check under the hood for the solid ink blocks (pictured) if you're not sure.

Step 3: Print on Pyralux

Note added 3-7-08: Use "high resolution" or "photo" mode when printing. This printer setting is typically available in the "Print Setup" menu of your graphics program. High-resolution mode prints more slowly and seems to promote better adhesion of wax to copper.

10 mil (250-micron) wide lines and spaces were printed from a Tektronix Phaser 850, which is an older model.

In most Phasers the copper side should face down when it goes in the manual feed and comes out upside down. Give a little push if the manual feed cog has trouble grabbing onto the sheet (more likely with thicker sheets)

Step 4: Etch It

Put the printed sheet in ferric chloride (copper etchant) for at least 5 minutes. Keep the etchant from getting on your eyes and skin. The etch time will depend on temperature, copper thickness and other conditions, taking up to 25 minutes, so keep watching for copper areas to dissolve and the polyimide film to show up. Bubbling with an aquarium pump, and heating up to 35-40 C will help the etch proceed faster and more evenly.

The remaining wax can be scrubbed off with a ScotchBrite pad and warm water, or isopropyl alcohol (rubbing alcohol). This can take some effort.

Step 5: Populate the Board

The flex PCB is now ready to cut apart into small circuits (if that's your plan) and to solder. You can tape it onto a piece of metal or a regular fiberglass circuit board to hold it steady while working on it. "Tinnit" nickel plating solution or similar can be used to make it easier to solder to, but the freshly etched and cleaned flex PCB is easily soldered as is.

Because it is a 1-sided PCB, without holes, it is most useful as a tiny cable or as a board for surface mount parts. Use jumpers if necessary for traces to cross on your layout.