I have a project based on the Global Historical Climatology Network data, going back 300 years.  This is based upon readings from actual thermometers on ground based weather stations, rather than tree rings or ice cores which are far more difficult to decypher.<p><a href="http://code.google.com/p/tempgraph/" rel="nofollow">http://code.google.com/p/tempgraph/</a><p>You can graph the data in various ways, all the way down to individual weather stations.