Last modified: 2006-05-25 08:08:51 UTC
{{#exists:pagename}} springs to mind. {{exists}} is not much used on en.wiki, since it's very slow (it parses the whole page just to establish whether it exists). I don't know how useful it is on wikipedias, but I know I need it to provide correct links and status in the workflow for translations (see requests linked from [[m:Translations]]).
Added an {{#ifexist}} ParserFunction in SVN trunk, r14375; documented at [[m:ParserFunctions]].