Here's a couple:

oldie but a goodie
String.prototype.pad
- allows you to specify a character (c) and pad a string to a certain length (n) with that character
Code:
String.prototype.pad = function(c, n) {
  var result=this;
  while(result.length<n) {
    result=c+result;
  }
  return result;
}
useless but fun
String.prototype.toRainbow
- returns HTML code for a rainbow version of the string by placing each letter in a differently colored span. Note: it would be pretty easy to modify this to create DOM nodes to be dynamically added into a document. The way it is now, you'd have to use either document.write or the hated innerHTML.
Code:
String.prototype.toRainbow = function() {
  var result="";
  var mode=0;
  var msg=this;
  var rr=0;
  var gg=0;
  var bb=255;
  ink=765/msg.length; //this ensures the whole string will go from blue to red to green back to blue
  for(var i = 0; i < msg.length; i++) {
    var theColor=Math.round(rr).toString(16).pad("0", 2)+""
                +Math.round(gg).toString(16).pad("0", 2)+""
                +Math.round(bb).toString(16).pad("0", 2);
    result+="<span style=\"color:#"+theColor+"\">"+msg.charAt(i)+"<\/span>";
    switch(mode) {
      case 0: //blue to red
        bb-=ink;
        rr+=ink;
        if(rr>=255) {
	  rr=255; 
          bb=0;
          mode=(mode+1)%3;
        }
      break;
      case 1: //red to green
        rr-=ink;
        gg+=ink;
        if(gg>=255) {
	  gg=255; 
          rr=0
          mode=(mode+1)%3;
        }
      break; //green to blue
      case 2:
        gg-=ink;
        bb+=ink;
        if(bb>=255) {
	  bb=255; 
          gg=0;
          mode=(mode+1)%3;
        }
      break;
    }
  }
  return result;
}