Are You Getting Bullied In Lane and Need Help?
Buy a 
Seriously, this item is really good for catching up since it a.) gives you the gold it farms b.) pushes your lane without putting you in danger or giving the enemy gold c.) provides defenses and a movement speed boost to make diving you really hard and d.) provides a movement speed boost that helps you get back to lane. Seriously guys, if you are having trouble in lane and/or are in a lane you know you won't win, consider buying a Zz'Rot and placing it under your Tower. It denies the enemy the chance of getting First Blood Tower Gold, gives you farm for free, and makes you harder to kill. Sure it costs 2700 gold and that can set you back some gold, but hey, if you are going to be behind anyway, you might as well be providing your team with some riskless pushing power. Hey? Did a butt ton of people come Top to counter your Zz'Rot under your tower? GREAT! You just made more than one enemy champion have to come deal with a lane that the enemy should be winning anyways! That in and of itself is a win! That's like having a Teemo need help against a Tryndamere. That's a win.
Champions you can rush this on without setting yourself too far behind:

(NOTE: Has mixed results when fighting Nasus, Sion, Veigar, and other stacking champions)
even if you're not losing, my friend does it and works really well

: "But what if we are not allowed to buy defensive items?"