The link to the entire data set (all version) is here.
The link to the SST fields from the current version (v2) is here.
For 1856-1981 this is the analysis of Kaplan et al.  which uses optimal estimation in the space of 80 empirical orthogonal functions (EOFs) in order to interpolate ship observations of the U.K. Met Office database [Parker et al. 1994]. The data after 1981 reperesents the projection of the NCEP OI analysis (which combines ship observations with remote sensing data) by Reynolds and Smith  on the same set of 80 EOFs as used in Kaplan et al.  in order to provide enhanced data quality of the former in the spatial resolution of the latter.
v1 and v2 correspond to the different versions of Reynolds and Smith, v2 being the current version (v1 has been discontinued; v1-based historical SST extension is provided here for the completeness only).
The base period used for the anomalies is 1951-1980. Climatological fields ("climate normals") with regards to which these anomalies were computed are those of the U.K. Met Office ATLAS7 GOSTA climatology, available here
IMPORTANT: If you do not care about statistical homogeneity of the long (starting pre-1981) SST records, and just need SST data after 1981, please use the original NCEP OI data from Reynolds and Smith (v.2). It is much better (1x1 degree resolution weekly based) SST analysis, which was degraded in the present product.
Kaplan, A., M. Cane, Y. Kushnir, A. Clement, M. Blumenthal, and B. Rajagopalan, Analyses of global sea surface temperature 1856-1991, Journal of Geophysical Research, 103, 18,567-18,589, 1998
Parker, D.E., P.D. Jones, C.K. Folland, and A. Bevan, 1994: Interdecadal changes of surface temperature since the late nineteenth century, J. Geophys. Res., 99, 14,373-14,399.
Reynolds, R.W., and T.M. Smith, 1994: Improved global sea surface temperature analysis using optimum interpolation. J. Climate, 7, 929-948.
This documentation has been last updated on: 27 April, 2011