Not Updated For Current Season
*Boots: Mercury Treads supplements well for CC heavy / caster based team, Ninja Tabi for physical DPS happy team, and if the team lacks all of this for some reason, Berserker Greaves
*Sell Doran's blade to get second Bloodthirster
Dying a lot?
Get a Frozen Mallet early, or just build the Phage before Last Whisper. Doesn't matter.
Is the enemy team Tank only?
Get a Madred's Bloodrazor after Infinity Edge.
Are casters giving you a headache and you need some MR?
Banshee's Veil works for most cases, but if it's extreme, a Force of Nature may be viable if it's all nuking. If you're facing a Ryze/Annie.. might be a good idea for a FoN. If you're facing Morgana, Banshee's Veil will probably save you from the ultimate.
Is the physical DPS hero (Olaf, Master Yi, Sion, Warwick) destroying you?
BUILD CREDIT TO LEIRKOV
Full guide: http://www.leagueoflegends.com/board/showthread.php?t=229981