Thats the charcoal canister.
It traps vapors from the fuel tank and when the car is running routes them into the intake to be burned during the combustion process. Personally I would leave it on. You won't really lose any weight by removing it and your fuel tank won't vent properly.
If you think it is an eyesore, and it is, you can always swap it onto the other side of the frame so that it is hidden in the fender well.