Humans - Human Perseverance +1
Warlocks - Fate of the Void +1 or with Improved Fate of the Void +2, Cumulative
Ring of Protection - +1
Bloodthread Armor - (0 or less = Bloodied) +2
Power that grants a Saving Throw with bonus:
[Must be either used on you from an outside source]
Cleric/Paladin
Channel Divinity: Divine Mettle - +Cha
Cleric:
Divine Aid - +Cha mod
Hallowed Ground - +2
Paladin:
Cleansing Spirit - +2
(Justicar)Just Spirit - Reroll
(Justicar)Just Shelter - +1 [When in range]
Fighter:
? Unyielding Avalanche: +1[Self] (Unsure, not certain what happens with stances when a person drops unconscious)
Warlord:
Shake it off - +Cha
? Hero's Defiance - Auto-Success (Unsure, Success = Stabilize or No Change?)
Battle On(Inspiring) - +Cha
Those are the only things I found so far.