has a useful blog and wrote a nice addition for prototype called event selectors.

 var Rules = {
   '#icons a:mouseover': function(element) {
     var app = element.id;
     new Effect.BlindDown(app + '-content', {queue: 'end', duration: 0.2});
   },
 
   '#icons a:mouseout': function(element) {
     var app = element.id;
     new Effect.BlindUp(app + '-content', {queue: 'end', duration: 0.2});
   }
 }

2006/03/28: Version 1.6.0 is out now! Get it while it's hot!

It also includes prototype V1.5 (pre1) which introduced a CSS like Elements selector. It's just called $$.

$$("#myTable td.even") -> will return all td cells with a class named "even" widthin an element with the id "myTable"

The only thing i'm missing is a way to select elements not just from the document root, but a given element (without an id)