The simplest, most environmentally friendly birdhouse is a snag--a standing dead tree.

You could also grow birdhouse gourds - they're not recycled per se, but they are recyclable (compost!), should only be used for one season, only require planting, drying and drilling a few holes (we get them as volunteers in our yard thanks to an original gourd several years ago - they don't need much care, at all), and are used by a lot of birds.

A simple wooden platform under an overhang for phoebes.

Or simply leave a door or window open to an old shed if you have one - "my" phoebes always nested in such a shed until I was forced to take it down by my insurance company.

In Britain you can usually find alot of waste wood from builders in skips. (These are the dumpsters that buildes put outside your house for rubbish when they are building an extension). You could need to saw it up and construct it your self though.
I've heard of plastic pipe offcuts being used, but again you would have to put ends on and drill holes, and they can get too hot and sweaty.
Old kettles or cooking pans for robins.
I read on another forum that someone had birds nesting in a supermarket carrier bag hanging in their shed which had a broken window!

Or how about modifying or joining together those baskets you sometimes get flower arrangements in, and making a hole in them.
You also hear of birds nesting in hanging baskets.

As a public service project I took a class to a school for mentally challenged students and we helped each assemble a birdhouse. We brought the materials with us. Each had a wooden board and 8" x 8" for the roof with two 1/8" wide holes drilled. The actual house was a One quart plastic flower pot with a hole we had already cut about an inch from the bottom and about 1 1/2 inches in diameter for the birds to go in and out. This was attached to the board by picture wire that went through holes on opposite sides of the top rim of the pot, through the two holes drilled in the board and twisted together outside on the top surface of the board. This wire could be used to hang on a branch of a tree. Everyone gained from this experience.

I've just read on another bird forum where they are using plastic bottle gourds in a nature reserve for purple martins.
I wonder if it would be possible to weave your own nesting/roosting pockets out of used supermarket carrier bags.

I like to make birdhouses but the simplest one is the one you buy. Pick it up, put it up, bam your done! Simple. I recently got a $20 bluebird house mail ordered from Audubon Workshop with a $25 coupon. Including shipping it cost me 98 c. Simple!! A screw in the fence post Bam, simple!! Done!

I guess the better question would be what kind of birds do you want to attract?

Juncos, chickadees, wrens, are the most likely candidates. I live in the woods.

Firevicar

Chickadees and wrens are cavity nesters and will use gourds. Or here's another thought - find a dead, hollow tree or limb, preferably one that has already fallen. Cut sections of it about 10" long, drill a hole into it, put a roof and floor on it, and hang on a standing tree. Make sure you baffle the box or extend the entry to deter predators.

Juncos usually nest on the ground or in trees, building their own nest so plenty of underbrush is all they need. Chickadees and wrens nest in cavities so you need to provide a box. The simplest wren house I made was with a flower pot. I cut out a round piece of plywood for the top of the flower pot which becomes the back. Drill holes using small masonry bits and screw the back on. Enlarge the drain hole to the proper size and it becomes the front entrance. Hang on a tree. Carolina wrens love pottery for some reason. You can always buy a simple nest box a walmart for $5-10 if you don't have the time or skills to make your own.

Juncos breed in the arctic. They come south (northern U.S.) for the winter. That's the case in the east, anyway--we have juncos all winter but never see them in summer. So you don't need to accommodate them with housing to use while breeding.

Juncos do breed as far south as Maine and throughout Canada, so they may breed in Washington state as well.

I get juncos in the winter here near Atlanta but they are gone by early spring.

Hanging baskets work for me. All I have to do is hang a few under the eaves and they flock to them to nest. I use airplane/spider plants. If you have a long haired cat or dog, brush or clip them on your porch during nesting season. Blow the hair into the yard and watch the birds grab it for their nests. If they're nesting in your baskets, you can tip them over and see the hair in the center of their nests.

The funniest place I've seen the birds nest was in our mailbox. Mailman forgot to close the door/ and when I went to grab the mail.. Mom let me know she was there !!It took about a week/ but she finially got the hint to leave.. the boys then put an old mailbox - minus the lid in the backyard, and sure enough... someone "used it" for their babies. :o)

I get the impression you think these ought to be available commercially?
or if you don't mind a bit of crafting
Try to find some 5 or 6" inch "jiffy pots". Not sure they make flower pots out of that mashed cardboard material in that size but if they do......
Jam 2 or 3 together to make sure its sturdy enough to last the year
Coat it with some wax for waterproofing
Close up and reinforce the open top with a tripled piece of this waterproofed cardboard stuff and attach to a structure that can't be climbed by racoons
Look up or call your local bird store to find out the size hole you want for the bird you are trying to help and fix up the opening side.
Or make a platform of the waxed reinforced mashed cardboard for platform nesters.
Recycle nest and all in compost pile at end of year.
Mary