Towhee is a Monte Carlo molecular simulation code originally designed for the prediction of fluid phase equilibria using atom-based force fields and the Gibbs ensemble with particular attention paid to algorithms addressing molecule conformation sampling.
Towhee made MC learning easy.
simple and intuitive.
Initially I found the size of the source code a bit daunting (I needed to do a few modifications, and add my own subroutine) but the manual provides really good explanations of the source code. Also, the code itself has clear commentary.