Which champions scale with kills in League of Legends?

It's not a widely used mechanic, but there are two other champions so far that can scale indefinitely.

Thresh's Damnation: passive allows him to collect souls from nearby slain enemies, either by walking near them, or with the Lantern from Dark Passage.

Veigar's Baleful Strike: Passive: Veigar gains permanent bonus ability power when he kills a champion from any source of damage. This stacks with the active ability's AP gain. Active: Unleashes dark energy at target enemy, dealing magic damage. If this ability deals a killing blow, Veigar gains 1 ability power permanently. This bonus is doubled for large minions, large monsters, and champions.

Veigar ability text quoted from the League of Legends wiki.