A powerful group of art-world volunteers, first assembled in 1968, are the ones who decide how much charitable deductions a museum donor pays, and how much estate tax is assessed on art.

They meet twice a year, and post their reports, including their names, in reports available online. (Here's FY 2012; there were 19 members.)

At their meetings, the experts consider each work and the taxpayer's appraisal without knowing whether it's for a donation or an estate. Then they offer their consensus, which—no surprise here—often runs counter to taxpayers', though not as often as you might think. Last year, the panel agreed with the taxpayers 51 percent of the time, in fact. At the same time, it also raised the values of 171 artworks in estates by two-thirds.