The datasets available through the Climate Engine tool and APIs are provided courtesy of dozens of data providers including NASA, the US Geological Survey (USGS), the National Oceanic and Atmospheric Administration (NOAA), US Forest Service, European Space Administration (ESA), European Center for Medium-Range Weather Forecasting (ECMWF), University of Montana / USDA-NRCS, University of California-Merced, Desert Research Institute (DRI), and many, many more.
Information about the datasets in the Climate Engine tool is available by clicking the yellow "?" icon next to the Dataset dropdown, as pictured below, which opens up a window in the tool.
For information about the datasets available through the APIs, visit the API Docs Datasets page.
Additionally, the Climate Engine Data Search table provides comparable information about each datasets that is available through the tools.