Ben Nolan's non-link element hovering trick for Internet Explorer (that normally only allows :hover styles on links) is pretty handy, but what really impressed me about it is his way of integrating arbitrary Javascript functions into the stylesheet itself:

body {
  background: url("
    javascript:
      document.body.onload = function(){
        ...custom js here...
      }
  ");
}

By specifying a rule like this in the browser's user stylesheet, it might even be possible to run a custom set of Javascript commands on opening of any visited website. (Although my brief attempt at making this work proved unsuccessful.) [via webgraphics]