The post below started as one exclusively about returnable bottles in Australia, but a little extra information has been added about returnable bottles from other countries.
What are these "returnable bottles"?
At one time, almost all breweries around the world had their own brand of returnable bottle or were members of bottle co-operatives that managed returnable bottles. Instead of being crushed to make new bottles, as happens in most cases today, these returnable bottles were returned to the brewery or bottle co-operative, washed, inspected for cracks and chips and returned to service.
These bottles, and those from the equivalent manufacturers in other states, are still sought-after by homebrewers because they are so sturdy.
Returnable bottles in Australia
I believe that West End and Southwark (South Australian Brewing, later owned by Lion Nathan/Lion) and XXXX (Castlemaine Perkins, also later now Lion Nathan) were the last beers to be bottled in the returnable bottles, some time in the mid-1990s.
Pickaxe bottles
Pickaxe bottles were made and managed by the Adelaide Bottle Company. All Pickaxe bottles have the Pickaxe logo somewhere on them. The logo below is from the last type produced and is on the neck of the bottle, where it is most often found on most 750ml Pickaxe bottles.
In these bottles the writing around the bottom of the bottle reads "Property of the Adelaide Bottle Co. P/L", in smaller writing than older bottles.
Older bottles have larger writing around the base than the newer versions and the writing reads "This bottle is the property of the Adelaide Bottle Co. Pty Ltd Thebarton". Note the wear on the bottle. I wonder how many times this bottle was filled, emptied, returned, washed and filled again.
The logo is very similar to those on the newest bottles.
A small number of my Pickaxe bottles are even older than this, and have a slightly different logo on them. Note the smaller hands holding the smaller pickaxe.
As well as the logos on the two bottles above being very slightly different (note the size of the hands), the text around the bottom of them is similar but not the same. They both read "This beer bottle is the property of the Adelaide Bottle Co. Pty. Ltd. Thebarton" (note the word "beer", which was dropped on the later bottles), but the font is slightly different and the lines of text don't line up the same.
I have at least one Pickaxe bottle that says "Southwark", instead of "Thebarton", which apparently puts it from some time in the late 1950s. I can't find this bottle at the moment but will post a picture when I come across it again.
An older but similar version of Pickaxe bottles was recently given to me by forum member pogierob. His father, also a homebrewer, collected dozens of the bottles over the years that had been discarded in the Australian outback.
These bottles look almost identical to the bottle in the picture above.

There is large writing around the base reading “This bottle is the property of the Adelaide Bottle Company Limited, Southwarkâ€

The most easily identifiable difference is a large PX embossed into their bases.

Looking closely at the logos in the neck of the bottle, they appear to be slightly different designs, with the most noticeable difference the larger hands in the image on the left. However, this may be simply a difference in the sharpness of the logo created from the mould.

At Winchelsea, near Geelong in Victoria, I bought a bottle that I assume is even older than the other longnecks I have. It has the logo on the base, rather than on the neck. It is slightly taller than the later bottles.

Here's a close-up of the logo.

I have seen some people assert on other forums that Coopers never used Pickaxe bottles. However, many of my Pickaxe bottles still have their original labels, and about a third are Coopers. (I also have some Sparkling Ale and Best Extra Stout that still have the original beer in them, from the 1990s!) The bottle second from the left is an older label and reads "Cooper & Sons Ltd", whereas the newer labels say "Coopers Brewery Ltd". The label on this bottle says that there's a 4c refund "at marine dealers in SA".
Coopers was the first SA brewery to stop using Pickaxe bottles, about 1996. The story goes that as Coopers began sending more beer interstate more of the bottles would never be returned, which was uneconomical.
The South Australian Brewing Company, which is now owned by the Japanese via Lion, used Pickaxe bottles until a few years after Coopers stopped, for its Southwark beers.
And its West End label.
Pickaxe bottles also came in other, smaller sizes. Here are a few that I have. On the far left is one of the 375ml "hand grenade" Coopers bottles. Next is a (full) bottle of West End "Eagle Blue", which was a light beer. The green bottle is from Southwark Premium Lager, which incidentally was a bloody good beer. On the right is a much older bottle than the other three.
Many of these smaller, stubbies (or "echos", as they are sometimes called in South Australia) had ring-pull seals. Here's the Eagle Blue ring-pull, still intact.
Here is some of the information about Pickaxe bottles, on a page from The Bottle Depot, which is a site specialising in South Australian bottles.
There is more information on the The Bottle Depot page, including when certain types of bottles were produced.The Adelaide bottle cooperative society was formed in 1897 when a number of brewers, producers and aerated water manufactures grouped together for better buying power of new and secondhand bottles and therefore share the associated costs. When propriety branded bottles emerged in the early 1900’s the Adelaide bottle cooperative society considered production of their own branded bottles.
In 1908 the society registered the Pickaxe trademark and its earliest trademark bottles were secondhand ring seal champagne type bottles that had the pickaxe trademark sandblasted or etched onto them and were introduced around 1910. In 1912 the Society changed its name to the Adelaide Bottle Company Pty and to protect their interests had bottles made with both society and company embossing and were used during the transition period from 1912-13.
[The information above is from] the book ‘The Adelaide Bottle Co.Pty.LTD-The first 80 years 1897-1977 written by F.B.Reed. [The book] states there is no significance to the placement of the hands on the pickaxe, but would occur [due] to the use of new molds where no instructions were given to the engraver as to the placements of the hands.
Returnable bottles in other states
At the time of the demise of the returnable bottle, each state had its own bottle company but the bottles were the same size and shape, with only the logo and text around the base to differentiate them.
Carlton and United Breweries beers in Victoria were distributed in bottles with the CUB logo on the neck and text that read "This bottle always remains the property of the Manufactures Bottle Co. of Victoria Pty. Ltd.".
In Queensland, XXXX was bottled in its own bottle.
On the neck was the XXXX man, and the base reads "This bottle is the property of Brisbane Bottle Exchange Co Pty Ltd".
There is a lot of information about different types of Pickaxe and other returnable bottles in this PDF from the Hidden Canberra website at http://canberracamps.webs.com.
The author, Ann Gugler, has kindly allowed me to reproduce the PDF on homebrewandbeer.com.
Returnable bottles outside Australia
New Zealand
During a recent beer-drinking trip to New Zealand I came across some returnable bottles.
The first was at the Stoke Brewery in Nelson. Both bottles were cider and dated from when the brewery was a cidery.

The writing around the base or bottom of the neck said “This bottle is the property of the Associated Bottlers Co Ltd NZâ€. The logos were just a simple ABC in a triangle.

At Bays Brewing, also in Nelson, there were some other returnable bottles.

The base read, “This bottle is the property of Harley & Sons Nelsonâ€
Here’s a Harley & Sons bottle with the label, for Nelson Breweries Pale Ale, intact.

Other countries
Some countries, including some in Eastern Europe, still have returnable bottles. This is a returnable Orval bottle, from Belgium.
I hope that at least one person finds this post interesting!
Cheers,
Oliver
EDITED
2014/07/16 to add information on PX Pickaxe bottles and NZ returnable bottles, and link to PDF from Hidden Canberra, along with a bit of a rewrite.