The email alluding to the 'Mike's Nature trick' has been perhaps the one most frequently quoted.
Background: temperatures during the past few centuries and millennia are reconstructed from the so-called climate proxy data, of which the rings of old or fossil trees are the best example. In some places, at high altitudes or high latitudes, the growth of trees is severely constrained by cold temperatures, so that in warmer than normal summers, trees tend to grow thicker rings or build wood of higher density, for instance. By applying statistical methods, the treering width can be interpreted in terms of past temperatures being above or below normal on those locations.
The rest is at klimazwiebel.blogspot.com