ESMValTool is a community diagnostics and performance metrics tool for the evaluation of ESMs that allows for routine comparison of single or multiple models, either against predecessor versions or against observations. The priority of the effort so far has been to target specific scientific themes focusing on selected Essential Climate Variables, a range of known systematic biases common to ESMs, such as coupled tropical climate variability, monsoons, Southern Ocean processes, continental dry biases and soil hydrology-climate interactions, as well as atmospheric CO2 budgets, tropospheric and stratospheric ozone, and tropospheric aerosols. ESMValTool is/was funded by several institutions and projects. The core development is carried out in DLR.
The software is open-source (see licensing).

--> Look in the natESM system table for more information about this optional component.

If you want to get started, the software is available on GitHub. The documentation is filed on ReadtheDocs.
There is also an online tutorial available.