Leet Text Bookmarklet. To use, make a new bookmark and then copy paste the code into the URL field, when made just click the bookmark with a textbox selected.

1337_5P34K3R.js

javascript:void(
function(inp,el,mp,k){
  el=document.activeElement||{};
	inp=el.value||'';
	mp={'is':/is/ig,'0':/o/ig,'1':/[il]/ig,'3':/e/ig,'4':/a/ig,'5':/s/ig,'7':/t/ig,'©':/c/ig,'Я':/r/ig,'м':/m/ig};
	for(k in mp)inp=inp.replace(mp[k],k);
	el.value=inp;
}());