Formerly, tree rings were best for this calibration: we know how old trees are because each year they grow a new ring. However, tree rings only go back so far, a little over ten thousand years. Now, Ramsey et al. (2012) in Science use sediments from Lake Suigetsu, Japan, to develop a longer record – organic sedimentation in the lake is seasonally distinguishable, with plant matter that drifts to the bottom of the lake being lighter colored in winter than in the summer. This record goes back to almost 53,000 years B.P. (before present, which is 1950 by convention).
This isn't really that interesting from a new science perspective – it's not new science at all. I find this particularly cool because I've spent the longest time arguing with young earth creationists, who have a deep loathing for radiometric dating, especially radiocarbon dating. One of the common "arguments" against the method is, perplexingly, that we don't know how much 14C was in the atmosphere in the past (and so how can the calculations be accurate?). But... of course we know! That's what the tree rings are for. And now we know how much 14C there was for a much longer period of time (middle graph; correct spelling is "Suigetsu," typo as appears in paper):
(Figure 3 from Ramsey et al. (2012), larger image here)
Climate science deniers are pretty bad, but creationists are, to steal Vizzini's words, "brainless, helpless, hopeless!"
This little "ha!" moment feels so good right now.