United States:(National Climatic Data Center). Contains monthly mean temperature and precipitation data that dates back to 1900: Time-Series trend data can be found here. The statistical climate information can be found here.
Monthly/Seasonal Climate Composites: (Climate Diagnostics Center). Allows one to generate maps that go back to 1948.
Northern Hemisphere Snow Coverage: (Rutgers University). Contains monthly and weekly data from November 1966 to the present (Downloads): Weekly Table and Monthly Table.
ENSO SST Indices for ENSO Regions: (Climate Prediction Center). Contains a monthly chart for data from 1982 to the present. A weekly chart is also available.
ENSO SST Anomalies (Select Data): (UCAR) TNI, R 1+2, and R 3.4. Contains data from 1871-Present. Reconstructed (From 1300 to near present). Historic ENSO Anomalies (hadISST) from 1870-Present: Regions 1+2, 3, 3.4, and 4 (ESRL)
Relative ENSO 3.4 Index (RONI): (NCEP). Contains seasonal data from DJF 1950 - Present
Monthly Atmospheric, ENSO, and SST Indices: (Climate Prediction Center). Contains a wide range of historical data.
Multivariate ENSO Index (MEI): (Climate Diagnostics Center). Contains graphs and a table comparing ENSO phases from 1950 to the present and a historic table that goes back to 1871.
Paleoclimatology: (NOAA). Contains a range of paleoclimate resources and data.
Quasi-Biennial Oscillation (QBO) Index: (Climate Diagnostics Center). Contains data for the QBO from 1948 to the present.
Sea Surface Temperatures: (NOMADS). Contains global sea surface temperature readings from 1854 to the present.
Sea Surface Temperature Anomalies: (Long Paddock). Contains global sea surface temperature anomalies from 1982 to the present. ESRL Plotting Tool (SSTA maps from 1856-Present) and ESRL Plotting Tool (SST maps from 1854-Present) ***
Southern Oscillation Index (SOI): (Commonwealth Bureau of Meteorology). Contains monthly SOI data from 1876 to the present (table and graph).
Standardized Northern Hemisphere Teleconnection Indices: (Climate Prediction Center). Contains the NAO, EA, EPO, WPO, PNA, and more in a table dating from 1950 to the present. Daily charts from July 1979 onward are also available. NAO from 1865 to Near Present.