Which countries, if any, offer on-arrival work visas?

There's precisely one case that I'm aware of: if you're a citizen of a country that has signed the Svalbard Treaty -- and you probably are, since signatories include most all of Europe, the US, Canada, India, China, Japan, Australia etc -- you are "allowed to become residents and to have access to Svalbard including the right to fish, hunt or undertake any kind of maritime, industrial, mining or trade activity", which includes working in any capacity.

Of course, there's a catch: Svalbard (Spitsbergen) lies at 74-81 deg N, a two-hour flight north of Norway, so it's pretty cold up there, and the population is under 3,000, which obviously means rather limited opportunities to actually find work. That said, it's a fairly big tourist destination so casual work is fairly easy to find in the summer season, but in winter there isn't much to do unless you're a coal miner. Or much to see, for that matter, since the polar night (=no sun) lasts from October to February!

Also, in order to get to Svalbard in the first place, you may need a Norwegian tourist visa so you can transit through to Longyearbyen.

Work visas aren't going to be given on arrival anywhere (I'd be highly surprised if they did). You'd be dealing with a lot of issues regarding taxation, potentially taking jobs away from local people, and benefits (or the lack thereof) - and Governments are starting to get really finicky about actual work visas as it is. Getting work visas, even working-holiday ones, involve an intense process that can be time-consuming and costly.

Most of the time getting some sort of work visa involves being sponsored by a specific company, and you're not really allowed to work anywhere else outside the scope of your visa. Having it be obtained on arrival would make this nearly impossible to do.

The closest thing I can think of is that when you come to Australia on an international student visa, after about a week or so post enrolment you can "upgrade" the visa to give you working rights (and even then they're greatly limited).

Just today I watched an episode of Border Security (Aussie TV show about customs) where two men on tourist visas were sent back to Portugal because they were found to have working documents on them, and the officials didn't accept the story of "I wanted to come to Australia to check it out first and see if I want to move here, the work documents were in case I do decide to return".

If you're Australian, New Zealander or Canadian (and possibly other nationalities but I haven't looked), you can apply for a Working Holiday Visa for The Netherlands (assuming you're eligible) after you have arrived. In fact, it is preferred over applying prior to arrival.

You are an Australian, Canadian or New Zealand citizen and you would like to spend some time (maximum one year) in the Netherlands in the framework of the Working Holiday Scheme/Program.

You travel to the Netherlands. You don’t need a visa to enter the Netherlands. If you want to work or study in the framework of the Working Holiday Scheme/Program you will need a residence permit, otherwise you will not be allowed to work without a work permit. Within 3 working days of your arrival you need to contact the Immigration and Naturalisation Service (the IND) for an appointment to apply for a residence permit (contact details: www.ind.nl ). The fee for this residence permit is 42 Euros, which has to be paid at the time of the application.

...

This is by far the quickest and easiest procedure. You can travel to the Netherlands as soon as you would like and you can start whatever you want to do there as soon as you have visited the IND.

Source - http://australia.nlembassy.org/services/visa/working-holiday-scheme%5B2%5D.html

If you are an EU citizen you don't need any work visa to work in a EU country

As an EU national, you're entitled to work — for an employer or as a self-employed person — in any EU country without needing a work permit.

There are some exceptions, for example:

Croatian nationals still face temporary restrictions on working in the EU.

I've obtained several distinct work visas in Germany, all were post arrival visas. I'm an American citizen, who visited as a visiting professor, but I thought most visa waiver partners qualify under most jobs that grant work visas.

You might find other German speaking or Scandinavian countries follow this rule as well, but always check with the consulate, maybe not all visa waiver partners qualify.

There is a slim chance of obtaining a post arrival authorization to work in one European country that shall remain nameless, but only under exceptional situations, and not reliably.

A priori, I'd expect Brazil offers your nationality exactly the same work visa options as your nation offers Brazilians.