I etched a PCB last night by ironing toner off of glossy photo paper onto the copper, then dunking it in HCl plus H2O2 etchant. It turned out quite nicely for a first go!
Process: (Warning: Hot stuff, corrosive stuff. Wear gloves, eye protection, an Afghan shawl, do not spill it on your oak floor, do not burn your Turin shroud with your hot iron. Be careful!)
Generate masks on Tesco Value Glossy Photo Paper from a laser printer; had to experiment some to see how well different toner (colours) sticks (for instance I found magenta > yellow). I ended up using the black mask, made using composite black for extra thick tonerisation. Check your laser printer & software is giving you the exact size you expect, your surface-mount pads won't tolerate everything being 2mm too big! (I checked this last of course, but thankfully they match 1:1.)
Cut the board to size, clean it well and polish it with Brasso (this will probably roughen the surface microscopically). [Ed: For future boards I found roughening more with sandpaper worked better.] With soap, make sure it's grease-free and dry it off. (No lint! Lint is evil!)
Copper side up, greaseproof paper over the top, heat it for a bit with a max-hot iron (clothes iron that is, you probably want to do this on a newspaper...). I heated it for about 30s, then lifted the greaseproof paper, then dropped on the mask paper (toner down). Be very careful not to move the paper at this point, so it doesn't smudge.
Greaseproof paper down, iron it evenly pressing firmly (without moving the mask paper) for maybe 30s again? Make sure it's all nice and hot, and even. The surface of an iron is not an even temperature, move it a bit.
Now you can cool the board, on a work surface -- put something flat on top so the paper doesn't curl up (it may warp when ironed).
Put the board and mask sandwich in a wet fold of greaseproof paper, and gradually get the mask paper soaking wet (a few drips at a time) whilst rolling it under a jar or rolling pin to make sure it stays flat. The paper will want to warp when it gets wet!
You will have a soaked bit of thick paper on the copper -- very very carefully remove it all by rubbing it with your finger; there will be a thin sheen left, and hopefully all the toner will stick. If it hasn't stuck firmly, GOTO step 2. This process can take a while, I brushed off the remnants of paper under the tap (with, er, a brush). End up with a board with only toner and no paper between the gaps. (Magnifying glass!)
(GLOVES AND EYE PROTECTION) Pop it in to etch. I used "a couple of capfulls" of 6% peroxide, plus "a capfull" of Swarfega Tough Clean Brick And Masonry Cleaner from B&Q. The latter is roughly 10-15% HCl. Swill it around for a while -- it'll probably take 15-20 minutes. Once the board started to show bare patches (IMG_3563) the process was very quick. Watch it at this point, so it doesn't over-etch. When all of the gaps are in all of the right places, pull it out with tongs and rinse it thoroughly under the tap. This board didn't etch all the gaps between the TQFP pads, but they can be scalpelled easily and leaving it in for longer to make those etch away would've destroyed other fine details elsewhere.
Remove the toner from the board with a rag and acetone. Good sources of acetone include a girlfriend who will have at least one bottle of nail polish remover which smells of peach and is disgusting to use, so it's up for grabs. The peach flavour does not help the PCB production process. This will clean off all the toner, and you can give the board a final polish/rinse/clean and you're done!
