CONCEPT (COsmological N-body CodE in PyThon) is a free and open-source code for cosmological N-body simulations on massively parallel computers with distributed memory. The code is written almost exclusively in Python, but it achieves C-like performance thanks to Cython.
For documentation on how to install and use CONCEPT, see1 guide.pdf.
The physics which CONCEPT strives to emulate, as well as the numerical methods implemented in order to do so, are presented in1 thesis.pdf.
1: By now, this is quite out-of-date.