While I don't use Showdown that much anymore, a command that I thought would be cool would be /calc.
This command should only work during a battle. When you use it, it would come up with a list of your current Pokemon's damaging moves and in percentage form, show how much damage each of its damaging moves would do to the opponent's Pokemon. You could also do something like /calc [enter Pokemon name], where it would do the same except for the Pokemon on your team mentioned in the command.
Just thought it would be easier than going to the normal damage calculator and putting all the details in.
