C'Thun is the Old God found in Silithus. He created the Qiraji, and supposedly took down a Titan.
Yogg-Saron is the Old God found in Ulduar, in northern Northrend. Ulduar was built entirely as a prison for Yogg-Saron, though he corrupted it's watchers and attempted to break free of his bonds.
N'Zoth is the Old God responsible for corrupting Deathwing. He is also believed to have a hand(or tentacle if you prefer) in attempting to corrupt the Emerald Dream. It is thought that he was the one that corrupted the Highborne into the Naga, and is supposedly near the Maelstrom, at the bottom of the sea.
Y'Shaarj was an Old God killed by the Titans. It is known that the Mantid worshipped(and apparently still do) him. It is believed that the Sha are the remnants of Y'Shaarj. It is also thought that he was the Old God whose death revealed exactly how deep the Old Gods' infestation of Azeroth was, which caused the Titans to imprison the Old Gods, instead.