In a web search I’ve discovered that the hottest stars are Class O stars up to 100,000 degrees Kelvin. Is there anything hotter?
I have no cite, but I think higher temps are reached in supercollider experiments. I suppose it might be for an extremely quick period of time or something.
Or I’m misremembering and completely wrong.
OK, did a Google search, and found this article by our own Bad Astronomer, Philip Plait.
http://www.madsci.org/posts/archives/aug97/872692372.As.r.html
No mention of a supercollider.
Phil gives a beautiful answer. But let’s make some assumptions about what we’re talking about – the high temperatures at the core of the Earth, for example, don’t count on “What’s the temperature of the Earth?” – Your answer to that question would give the normal ambient temperature range for the surface and what the median or mode temperature is. Likewise, the million-degree temperature of the solar core and corona (see the Uranus-magnetosphere comment) are irrelevant – the “temperature of the sun” in everyday usage is the ~6,000 K surface temperature of the photosphere.
Given that, the extremely high temperatures of class O giants and Wolf-Rayet stars are the highest temperatures for normal matter that can be observed, as opposed to rapidly-moving rarefied gases, stellar cores, and such.
The surface temperatures of pulsars, though, are probably enormous, though there’s no effective way to measure them AFAIK – but if you think about red-hot, white-hot, blue-violet-hot, ultraviolet-hot, as an ascending spectrum, the pulsars stars are X-ray-hot. What that translates to in K I am not sure, but I’d suspect it’s in the hundreds-of-thousands or millions of degrees.
Keira Knightley, perhaps…
If you can observe them directly, you can measure their temperature. And young pulsars (the Crab pulsar, for instance) are, in fact, exceedingly hot (millions of kelvins sounds right, off the top of my head), but they’re continually cooling, so old ones could in principle be arbitrarily cold.
It remains to be seen, however, how hot Ms. Knightley will be as she ages. My estimation is that she’s currently in a T Tauri stage, and will become hotter yet once fusion sets in.
She’s obviously a “K” star, though, so she may cool way off as she settles down for the long haul. 
So, are any of these examples hotter than black hole Accretion disks, some of which are millions of degrees K?
I bet a pair of singularities…a black hole and a white hole…would emit wavelengths of mere Planck Lengths; that would be very hot indeed.