What - you've a big dangly thing that your chain runs through hanging below the rear axle, that you can move when your legs cry out that "life's not fair, it's too hard.."? Don't worry - you've got GEARS, and non of this applies to you.
Go sit down on the naughty step and think about what you've done *.
However, if you've set up your tent in 'Camp ore-sum' because you ride a single speed, and have horizontal dropouts, and you have girder bending thighs of steel (of course you do) then you'll need a chain tug...
I've chosen to write a little report on the various chain tugs I've bought and used for my XC/race singlespeed bike (on-one scandal) because I've been through a few..however, if you're short on time, to save you my "buy cheap, buy thrice" approach, the answer is buy a Surly tuggnut!