Is there a powerstrip that accomodates most if not all AC power plugs?

It's very hard to make a universal socket that's compliant with all the safety standards that would apply. Eg in the UK BS 1363 sockets must have shutters over the L and N holes and plugs must be fused.

The simple answer, if you are travelling to many countries, is often to take your domestic power strip but change the plug on it to a IEC 60320 inlet. This will work with C-13 and C-15 plugs, and almost everywhere will have a kettle lead or a computer lead you can borrow.

If you are expecting foreign visitors, provide a IEC 60320 multi-way strip with C13-C14 leads for most appliances and C-5 plugs for laptops.

I think they're more common in countries with poor safety regulation of electric devices.

This adaptor strip/board was at my hostel in Tbilisi, Georgia at the time the question was first asked:
^{It seems to be made by a company called "Veto" with model number 606F, but I couldn't find anything by Googling.}

And a few days ago I found a "bin" full of these in one of the very few supermarkets here in Vientiane, Laos:

^{Chinese labelled imports.}

Now that I've spent a month travelling through China I can say without doubt that it is the golden country for these devices. There are a myriad variety of them on sale everywhere. These photos are from a supermarket, not an electricial or electronics store, and it's in a city most Chinese haven't even heard of:

^{Typical range in a typical supermerket anywhere in mainland China.}

^{The one I chose was compact and seemed to be a decent quality. It has two sockets for two- or three-prong plugs and one for only two-prong plugs. The three-prong sockets accept Australian style plugs and Europe/India style plugs. They also accept a style with two flat prongs that works in Taiwan and probably other places. The plug on the other end of the adaptor is Australian and Chinese-style three-prong.}

Yet I don't think I've ever seen something like this in Australia. We have no shortage of supermarkets and electronics shops with tons of adaptors and powerboards but we have quite stringent safety standards on electrical products.

One of the most important things when buying power strips is safety! I do not see anyone mention this. There are two different safety issues here:

1. Safety of the place, for example overloading. Last thing you want to do is burn the hotel room or so. Make sure it can provide power for more devices and in case of any abnormality it should be equipped with circuit breakers.
2. Safety of your devices, in many countries power is not as stable as in EU or USA. A power strip with Surge Protector to protect your devices is very important.

In addition to above, it is universal and can handle UK, EU and USA and almost all Asian and African plugs. It also has two USB outlets to make it easier for you to charge.

From Amazon.

I do see your point about the collaboration among people of different power types:

(Yes, the rightmost plug is going through three adaptors. It's from 5 years ago, but I believe that one is mine. The picture was taken in South Africa: SA to UK adapters and UK to US adapters were plentiful, other combinations not so much. After that trip I started carrying the mini power strips.)

But surely when you're in your hotel room, all the things you want to plug in are the same kind of plug? I take a small power strip (my smallest is a cube that accomodates 5 plugs; my airport friendmaker) and plug it, using the adapter, into the wall. Then I can plug everything else into the power strip.

I never bother taking a stepdown transformer with me because the only things that I charge (eg my laptop; battery recharger; phone) on the road are happy at both 220 and 110 (and certainly within 10% of each of those.) But if you did then you might want two small strips. Something like this won't take up much room at all:

Multi-plug sockets exist (I've sometimes seen them in hotels), but you still have to be careful what you plug into them, as there's also a difference in voltages.

While many power units, especially those found in travel-related gear such as laptop computers, accept 110V as well 230V, not all do. A 110V device plugged into a 230V socket is not going to live long.

Maybe multi-plug sockets are not common because makers (and venues that might deploy them) fear getting complaints/lawsuits when this happens.

Any multi standard socket is unlikely to properly comply with the standards for most of the plug types it accepts. In particular it's unlikely to comply with the standards of the country it's sold in for their own plugs (being noncompliant with a countries own standards is far more likely to be a legal problem than being noncompliant with some random foriegn standard).

There seems to be an acceptance that travel adaptors will have lower safety standards than normal plugs and sockets. Most of the ones I see sold in reputable outlets in the UK have a warning on them along the lines of "not for long term use". The ones sold for people travelling from the UK typically have a warning along the lines of "not for use in the UK" while the ones sold for visitors to the UK will typically not accept UK plugs. I suspect safety regulators regard travel adapters as a "nessacery evil".

As others have pointed out multi-standard power strips do exist but they are mostly sold in countries with weak electrical safety regulations. In the UK they are only available from dubious sellers. Typically via marketplace sites where the legal seller is seperate from the site operator.

Things I would consider if running an event.

1. Provide multiple types of power strip. In the UK for example I would consider providing schuko power strips with british plugs. I would be more reluctant to provide "wrong voltage" strips though.
2. Provide power cords. Most laptops have cloverleaf inlets, most of the remainder will have IEC or figure eight ones. Would a bucket of each type of lead really break the bank?

Yes, I have one which I use at home, bought from Amazon (UK). It accommodates UK, N. American, continental European, and possibly others too. I find the sockets were a bit tight to start with for UK plugs, but now I've used it for a while, I find it invaluable.

1. None of the above addresses the problem of Schuko sockets:

This is recessed and as such, doesn't work with the above. Skross for example has a series, any of these combined with the multisocket power strips above solves the problem http://www.skross.com/en/category/18/europe-input.html . Alternatively you can get a ground collar which passes the two pins of the Schuko plug into the multisocket and changes the recessed ground into a protruding ground leg:

Finally, Wonpro seems to have a universal power strip which also allows for Schuko plugs but I've only found it on Chinese wholesale sites:

If you do not need grounding to work then currently nothing beats the MOGICS Bagel. You get 4 universal sockets, 1 US socket and 2 USB in a small package and the sockets do not block each other. The third legs of grounded plugs are not blocked but they are not connected to anything.

The universal plug adapter you get for this one is so small it fits inside the Schuko socket (be careful not to lose it, the routine scan I do every time I leave will NOT catch it inside a black socket like the one in the Amsterdam KLM Lounge where I left mine) and yet it's EU/UK(!)/AU. There are hidden surprises in there: the bottom of the Bagel has a little notch so even if you engaged the third UK leg you can still put the adapter back in the middle and plug the whole shebang in.

Wonpro makes a bunch. Here's one I have -- a 3-prong one as resold by Ceptics. (There's a really cool version with a standard C13/C14 plug in the back, but I haven't managed to find a reseller yet.)

If you're in a country that doesn't allow for unlicensed power adapters, you can order some from China's AliExpress:

Most options are under $20 with shipping and include several USB connectors.