I like [LiE][1] for computations with Lie algebras, especially roots and weights computations, plethysms,...  It also has a [web form][2] interface.


  [1]: http://www-math.univ-poitiers.fr/~maavl/LiE/
  [2]: http://www-math.univ-poitiers.fr/~maavl/LiE/form.html