Ryze is an excellent candidate for use with the Frozen Heart, since the damage of his abilities is increased with his maximum mana. The added mana and cooldown reduction also gives him extra sustainability in lane and the armor reduces physical damage taken.
Warwick is also an good user of Frozen Heart, as the mana helps compensate for his mana troubles, the armor will make him much more durable and the aura will improve his already awesome dueling.
In general, this item is good on:
Tanks and bruisers that are generally sitting on the middle of enemy team during teamfights, as they make the best use of the aura due to this (by affecting as much enemies as possible); and they are generally mana hungry. Examples include Maokai, Amumu, Wukong and Sejuani.
Champions in general, that work well with cooldown reduction, have mana problems, and make very good use of the passive for extra dueling potential. Examples include the aforementioned Warwick, as well as Nasus, Irelia, and Olaf, as well as some assassins, such as Kha'Zix.
Even if you don't need to slow the enemy's attack speed, the extra armor, mana and cooldown reduction may come handy against every AD champion.
This item can also be bought by manaless champions, should they wish the combination of cooldown reduction, armor, and attack speed reduction in one item. It may be better to buy Randuin's Omen and/or Thornmail, though, as the mana from Frozen Heart is completely wasted.
For its combine cost of 650 gold, Frozen Heart gives 150 mana, 30 armor, 10% cooldown reduction, and an attack speed reducing aura.
Frozen Heart helps counter DPS carries like Aatrox, Tryndamere, and ADCs in general, since they all rely on auto-attacks and the decreased attack speed will reduce their damage output.
Owners of Frozen Heart benefit greatly from the Oppression mastery, always having 3% damage reduction from all enemy champions in 700 range.