Poll with comments: skill tree.

The damage calculator and the guard strength calculator are much more "cheaty". IMHO.

Damage and guard strength can be calculated in your head — for example, by remembering an object's value (Dragon Fly Hive 15000 — on Diamond, SM, strong monsters), but you can't calculate the skill tree in your head. Regarding calculators: it's better to learn to calculate yourself — you'll train your memory and get more enjoyment from the game.