Youmuu's Ghostblade on Fiora. Sometimes I build it before Tiamat. It absolutely destroys champs and decimates turrets. The movement speed from the active is great for pushing lanes faster, dishing out more damage, and the movement speed is great for running away when you cut it close. Once you have Hydra with it... oh gawd... heavenly sword damn.
Feral Flare on Shyvana. Going defense seems like such a waste on her early on, especially considering she has no CC for ganks. I alternate what I build offense/defense as I go to stay balanced, but cannot go without Flare. The early damage is great. It synergizes amazingly well with her kit. I also feel like I get way more gold than I do with Ancient Golem or even Elder Lizard (though I'd never build it on her.) I haven't actually done the math, but it feels to good to just be in my head. It also saves me from having to drop money on BotRK as Flare already provides atk speed. I build Hydra instead which helps dominate lanes when I come in. Add in the Sunfire Cape after that and we are a GO! Captain enchant on boots turns me into a split-pushing beast late-game.
Runaan's Hurricane on Jinx. I typically only build it if I'm already ahead, but when I do, it's scary what Jinx can actually do with it. The attack speed is nice, but the aoe with her rocket in teamfights is why I really like it -especially if I already have high-damage items. It's a fun win-more for me.
Morellonomicon on Lulu. I love everything about it and it compliments my aggressive Lulu play perfectly. It solves all my mana problems by itself and gives me a nasty damage passive that I use on Warwick frequently. I usually rush it after Sightstone. It's funny, most of the time I pick Lulu, there's a WW on the enemy team to burn down. :P HA!
Essence Reaver on Hecarim -even more so now that the price is lowered. I did a lot of math comparing it to Trinity Force and Essence Reaver suit my purposed far better early game than Trinity Force does. The plain and simple: Sheen and Trin are damagemultipliers whereas ER gives you flat AD your skills can also benefit from. Also helps the crappy mana regen and dmg he has early game too. I love everything about it. The new 3200 price makes it comparable to Trinity Force now depending on what you're looking for between the two. I've had much better success with Hec as a result.
Essence Reaver on Graves. I usually build BT or IE after. The mana regen is so nice as it helps me spam skills more which keeps the lane a pain-in-the-ass for anyone I'm up against.
Also, Mercurial Scepter on everyone I've listed other than Lulu in the right situation. Cleanse is extremely underrated. I tend to build it more often on Riven than anyone else, but it's good all-around for dealing with CC. Put that on Riven and tell me you're not the stuff of nightmares.
I love playing with builds! <3 :)