Earth System Documentation (es-doc) provides metadata obeying CIM standard (Common Information Model).
The es-doc homepage offers services for metadata search, comparison and creation. For users, especially metadata search is beneficial. One of its strengths is subset search. For example, metadata can be found about the MPI-ESM_LR model in combination with CMIP5 experiment "decadal1960": initial conditions, simulation and platform details. The metadata have been collected from data producers and modeling groups.
By the way, CIM standard and tools were originally developed in the former Metafor project. The es-doc homepage is therefore sometimes called "CIM portal" or "Metafor portal" but they are all the same.