Wit's End on Warwick needs no mention.
In particular, it works well on ranged heroes that have opportunities to poke. It's situational, but extremely useful vs. heroes with low mana pools, or heroes that run in extremely delicate mana balances; I just played a Tristana game where I rushed Wit's End as my first item because I was soloing vs. Heimerdinger middle.
Any time he ran in to Rocket or Grenade, I just sat through it and auto attack burned more of his mana than he used to cast the spell to begin with. Whenever he was low mana, I made it a priority to hit him occasionally just to keep his mana pool too low to cast any spells. It's doubly useful against casters in that it has magic resistance not only in the item, but the build up as well.
Edit: That game also had an Alistar. Any time he would pop his ulti, I'd run back to the outskirts, pop Rapid Fire, and white hit him all day. He'd run out of mana quickly and be completely unable to Heal after the duration, affecting his entire team as a result.