I have used MCMC to estimate the value of a parameter $\theta$ from some data. I have thousands of samples from the (marginal) posterior distribution. The distribution of $\theta$ is roughly Normally distributed around zero. I can calculate the mean/median/standard deviations/quantiles/etc., no problem. What I am wondering is: how to calculate an upper bound on $|\theta|$? (Perhaps for a given confidence level?)
From your comment: Yes you can just take the absolute value of all the samples and find x such that the CDF at x is 0.95. This is a one sided credible interval. By the way you should just be able to track the absolute value of your parameter in what ever software you are using (winbugs for example) so you don't even have to take the absolute value of the sample. Looking at the posterior density of the absolute value should help you see why this is the value that you are looking for.