Rei's, Macro, OptiFine.
That's all I really know. I think we used to have a list of approved mods.....we might not anymore, or its floating around the forums somewhere.
Anything that gives an obvious advantage is obviously illegal, but any mod that isn't on the approved list (If it's still around) can be technically be called illegal. Id stay away from using anything that isn't Rei's OptiFine or Macro Mod