Since the hockey stick paper in 1998, there have been a number of proxy studies analysing a variety of different sources including corals, stalagmites, tree rings, boreholes and ice cores. They all confirm the original hockey stick conclusion: the 20th century is the warmest in the last 1000 years and that warming was most dramatic after 1920.
Debates continue even now over the statistical methods used in Mann's initial study. However since 1998, there have been at least ten proxy studies, analysing a variety of different sources including corals, stalagmites, tree rings, boreholes, ice cores, etc. Here is a visual summary (courtesy of Global Warming Art) of the various results (solid black is actual observed temperatures):
The results all confirm the same general conclusion: although each of the temperature reconstructions are different (due to differing calibration methods and data used), they all show some similar patterns of temperature change over the last several centuries. Most striking is the fact that each record reveals that the 20th century is the warmest of the entire record, and that warming was most dramatic after 1920.