@All, I can only give you some examples. In most areas in physics, in the USA, the postdoc salary ranges from 36000-45000US$ for most universities. Some high-ranked universities pay as much as 60000$ too to their physics postdocs, with an exception of Simons Center which pays 70000$. On the other hand, national labs pay around 70000$ for their physics postdocs - I don't know why the difference between the university payscale and the national lab payscales in the same country for the same field! In the UK it is fairly uniform for any field, at least in science and engineering. It is usually between 29000GBP to 33000GBP per year. Where you lie in this range depends on how many years you have passed after your ph.d. In Australia, they usually pa 60000 to 70000AUD + 9 (if the contract is for short term - I don't know if the short term means 1 year or less than 3 years though) or 17% (if the contract is for 3 years or more) superannuation, i.e., retirement fund. Again, there are precise rules on where you lie in this range depending on years after the ph.d. date.
In South Africa, it is somewhere between 180000-240000 Rand per year tax-free.
In New Zealand, you may expect the salary around 50000-60000 NZ$.
In Germany the salary levels seemed complicated to me when I was applying as there are many kinds of taxes and you may avoid some if you are married and have child etc.
In Brazil, you may expect around US$40000 (I don't remember what the exact amount in the Brazillian currency was) or even more if you have the national postdoc fellowship.
If you ask me, the postdoc life is miserable if you are in most universities in the USA and have even a small family to support. In my experience, Australia or the UK where the salaries are uniform and above the national averages for the fresh engineers (or other professionals), your life can be much more pleasant - well unless you are in the expensive area in Sydney or London!
Brazil seems excellent too but I can't speak Portuguese !
Edit: Forgot about Ireland. The salaries are around 35000Euro per year. I used to get paid around 43000Euro per year (each every expenses like insurance, taxes, 'levy' which was another kind of tax, etc. were on me though) in 2009-2010, however I would think the salary levels have gone lower after the credit crunch in which Ireland has been affected the most.
I should also mention that most postdocs in Math departments in the USA are paid 48000-55000US$ per year but they have to teach 2 courses a year or so whereas the physics postdocs don't teach at all (if they teach then they get more money than their salary).