Not Updated For Current Season
Short and simple Off-Tanking with Renekton.
NOTE: This is not an overly in depth guide(OK this isn't even in depth at all).
Pros / Cons
Strong Early/Mid/Late Game
Level 6 Ulti can be used to bait enemy's into a fight they think they can win.
Cons(If your not tanking):
Easy to chase/escape from
Armor Pen Red: Great for early and late game damage dealing, remember your off-tanking you still want to be able to put out a decent amount of damage.
Flat Armor Yellow: Helpful for early game gank survival and pads your armor a bit for late game.
Cooldown Reduction/Attack Speed Blue: First off you benefit greatly from both of these, because your ability's are powered up rage and hitting things generates rage the faster you hit the faster your gonna get rage. If you go the cooldown route you will be able to cast your damage dealing ability's more often which is always a plus especially when your ability's heal oyu and reduce armor of enemy's you hit.
Armor Pen Quintessences: While you could go for CD/AS/HP the flat damage increase that all this armor pen will give you is far better than any of the stated(Remember your ability's and attacks deal physical damage).
Have a tank already = get the CD Boots, no tank = start with Merc Treads.
To be honest item builds are situational as hell so following this build wont get you a win every time. You have to get a feel for all the items and see what the enemy team is doing then compensate.
Tips for item building:
You want AS/ArP/AD for your damage items. AS to speed up your on hit effects and to build rage faster. ArP to slice through high armor opponents and crush squishy's. AD to increase the damage your ability's do for increased Health regen.