The Loopmaker is a new script at AE Scripts that "loops any layers, footage or comps with one click. The script will loop any amount of selected layers from the furthest in-point and the last out-point by pre-composing and doing a dissolving loop. It will then apply an expression to the looped comp so that it can be stretched infinitely from either the in or the out-point. There is an option to use the Blend effect instead of using opacity to make the dissolve."
AE Help shows a few other ways to loop footage and other items; other resources include the script pt_AutoExpress and a tutorial from Motionworks: Looping a Precomp.