Some ideas: Hang the empty seed packet over a stick shoved into the ground on the end of a row.
This will usually fade by the end of the season, but at that point the plants have grown large enough that you know what's what.
Use blank dog tags and a stamp set to create metal labels.
Write in pencil on the back of strips of scrap vinyl siding .
I've seen small wooden signs nailed to stakes that have the plant name painted on them.
Similarly, I've seen signs that appear to have been woodburned/carved.
If you want really cheap and easy, don't mind appearance much, and are willing to perhaps replace them halfway through the season, save your junk mail.
Cut envelopes open on the short end, write your label on the back with a sharpie and hang it over a stake as mentioned above for seed packets.
Write with a paint marker on an empty wine bottle , and invert the bottle over the end of a stick.
Carve the name into an flattened-out aluminum can .
Cut open cardboard milk cartons and write on the inside with a sharpie.
The plastic coating on the inside of the cardboard makes these very durable; they'll probably last a couple of seaons -- perhaps with a refresh on the marker.
Not a direct answer, but I was reminded by @Rob Forrest's suggestion that I should mention I've had good luck with planning my plantings so that I have a map of what is supposed to be where.
Of course nothing ever goes exactly to plan, so I make sure to update the map after reality strikes
and then I have a record of what is growing where.
gardening.stackexchange.com/users/51/bstpierre
I slice up plastic containers/tubs, such as one-litre yogurt tubs, with ordinary scissors or kitchen shears, into strips about half an inch across and 6 inches long, and I write on them with a grease pencil.
Just the plant (eg lettuce, green beans) - the specific type etc are recorded in the garden book, not on the label.
I write in the top half of the strip and then just stick it in the ground.
I've had the same grease pencil for over 20 years.
I may need to replace it in the next decade or so.
I keep the tags in the fall and reuse them if I am planting the same stuff later (and most of us usually do plant the same core things all the time.)
Where I live, we can recycle those tubs, so any strips I don't want any more can go in the blue box.
As an extra bonus, some of them are written in the "brand new to writing" handwriting of then-small children (now grown), giving me a little nostalgia as I look through them to find the one I want, a bit like putting kindergarten-made ornaments on my xmas tree.
gardening.stackexchange.com/users/754/Kate Gregory
One grower we buy from uses cheap PVC Venetian blinds.
You can cut the slats with scissors and they are virtually indestructible.
Label with grease pencil or indelible marker
and you are good to go.
This worked well for me as we had a few sets of these blinds in the garage and it was better than throwing them out.
gardening.stackexchange.com/users/499/kevinsky
Used plastic forks and knives, save them up from lunch, then write on the black ones with silver sharpie and the white ones with black sharpie...
Re-using garbage is awesome.
gardening.stackexchange.com/users/1158/Grady Player
I heard a great suggestion on Gardeners question time.
Instead of sticking in a label that invariably gets lost, fades so that you can't read it, or just looks plain ugly; why not take a photo of what you've planted and name that photo with the name of the plant.
You'll have a record of exactly whats there.
gardening.stackexchange.com/users/1241/Rob Forrest
I used popsicle sticks and a sharpie for plant labels this year.
It worked really well for all of our seed starts.
If you want to save time, you can color code them so you don't have to write out the same thing dozens of times.
gardening.stackexchange.com/users/6/Doresoom
If you have open fires in your home you're likely to have a supply of kindling/starters.
If so, you're either buying them or, better still, making your own out of old pallets/demolition timber.
They make great plant labels.
In my case I just rob from the kindling stack whenever I need labels.
Easy to write on with a Sharpie.
Return them to the kindling pile when they're old and worn and faded at the end of the season.
gardening.stackexchange.com/users/99/Tea Drinker
I would like to put small plant labels in my garden so that I don't forget what exactly is sown where.  Can you suggest plant labels that:   are made from non-toxic materials  can have arbitrary text written/carved in them  are relatively easy to make  are made from materials easy to obtain  are made from materials that are durable outdoors for at least one growing season
How do I make plant labels for outdoors?
gardening.stackexchange.com
<plant-labels>
gardening.stackexchange.com/questions/4278
gardening.stackexchange.com/users/1186/nettle
Hang the empty seed packet over a stick in the ground. You can also use blank dog tags, scrap vinyl siding or a flattened-out aluminium can.  The simplest idea is to hang the empty seed packet over a stick shoved into the ground on the end of a row or you could do the same with an envelope or a cut up milk carton. Some more interesting ideas include using  blank dog tags and a stamp set  to create metal labels, scraps of vinyl , flattened aluminium can or small wooden signs nailed to stakes. Or why not write with a paint marker on an empty wine bottle , and invert the bottle over the end of a stick.  
The simplest idea is to hang the empty seed packet over a stick shoved into the ground on the end of a row or you could do the same with an envelope or a cut up milk carton. Some more interesting ideas include using  blank dog tags and a stamp set  to create metal labels, scraps of vinyl , flattened aluminium can or small wooden signs nailed to stakes. Or why not write with a paint marker on an empty wine bottle , and invert the bottle over the end of a stick.   I slice up plastic containers/tubs, such as one-litre yogurt tubs, with ordinary scissors or kitchen shears, into strips about half an inch across and 6 inches long, and I write on them with a grease pencil or save up used plastic forks and knives, then write on the black ones with silver sharpie and the white ones with black sharpie...