a plugin that lets you condition parts of the page depending on GET variables transferred with the url.
the plugin has three functions. the first of which, chs_if_urlvar, lets you condition that if GET variable designsated is equal to the value designated, the enclosed text will appear (an example is in the documentation).
another one, chs_urlvar_default, will show the enclosed text only if the variable is not declared in the url, and a third one, chs_urlvar_exists, will show the enclosed text only if the variable is declared.
more info about usage is in the docs.