MrBayes is a program for the Bayesian estimation of phylogeny.

Bayesian inference of phylogeny is based upon a quantity called the
posterior probability distribution of trees, which is the probability of a
tree conditioned on the observations. The conditioning is accomplished
using Bayes's theorem. The posterior probability distribution of trees is
impossible to calculate analytically; instead, MrBayes uses a simulation
technique called Markov chain Monte Carlo (or MCMC) to approximate the
posterior probabilities of trees.

WWW: http://mrbayes.csit.fsu.edu/