What temperature can you attain with a solar furnace? A solar furnace is a device that concentrates the sun's light on a small point to heat it up to high temperature. One can imagine that in the limit of being completely surrounded by mirrors, your entire $4\pi$ solid angle will look like the surface of the sun, at about 6000K. The target will then heat up to 6000K and start to radiate as a blackbody, reaching thermal equilibrium with the sun. 
The question is: is there any way to surpass this temperature, perhaps by filtering the light to make it look like a BB spectrum at higher temp, then concentrating it back on the target?
 A: Theoretically the answer is yes. That is because the sun is not a blackbody emitter, there is an excess of UV radiation. So if you were able to achieve radiative equilibrium with only UV light (which is maybe 1% of BB radiation at those temps), you could do it. Practically, I'd think it would be just about impossible, as your filter would have to have it's innermost surface at nearly 6000K.
Note: The solar UV primarily comes from the chromosphere and corona, which is heated (in some not too well understood way) by mechanical/magnetic energy derived from convective processes. The X-ray excess is even greater than the UV excess. Even the earth gives off detectable gamma rays, and that would be impossible thermally.
A: According to wikipedia, it can reach 3500-4000 °C
You can increase the temperature by  choosing material which is black at visible light range and white at IR range. 
A: There is no limit to the degree of concentration. In theory, the entire output of the Sun could be concentrated into a small point.
