ChatReplace is plugin for the Minecraft wrapper Bukkit that provides a method for administrators to reactively modify chat on a server through matching regular expressions. You could use this to match undesirble words and change them into less offensive ones or censor them completely. Alternatively you could use the plugin to create simple roleplay themed events or enforce a particular style of language on a server.
Before installing, you need to make sure you are running at least the latest recommended build for Bukkit. Support is only given for problems when using a recommended build. This does not mean that the plugin will not work on other versions of Bukkit, the likelihood is it will, but it is not supported.