Reversible-jump MCMC
The Reversible-jump Markov Chain Monte Carlo (RJ-MCMC) algorithm is an extension of the Rosenbluth-Metropolis-Hastings algorithm where the number of dimensions of the parameter space is allowed to change.
And example of application (from Green 1995) is
the multiple changepoint model for Poisson model where the rate is supposed to be piecewise constant but changes an unknown number of times.