FESOM is a core component of the German natESM.

FESOM is a sea ice ocean model developed by researchers at AWI in Bremerhaven. It works with flexible meshes to achieve increased resolution in dynamic regions using finite element methods. FESOM allows global multi-resolution simulations without traditional nesting. Nowadays FESOM2 is the actual version, which uses the finite volume method.
The software is open-source available.

There is also a special version for coastal applications FESOM-C.

More information, tools, publications and media about FESOM here and in the Reasearch Software Directory of HIFIS as well as  impressive videos from results.

If you want to get started, the latest stable version is available at GitHub. The documentation is filed on ReadtheDocs, which also includes a section "Getting started".