I would like to use CSS3 selectors (such as :nth-child), but they aren't supported by any version of IE. In my Rails app I've already included Script.aculo.us and Prototype and it would be great not to have to include Jquery.
Is there any script to fix IE's behaviour? (I'm not interested in IE6 compatibility)
Have a look at the IE8.js library by Dean Edwards. It does pretty much what you're asking for.
See the IE7 Test Page for the things it fixes.