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Some features that grant advantage may be missing from this list, as they would theoretically be part of the linked Q&A: "How else can I get Advantage on Death Saving Throws?"

Class Features

Artificer:

• Flash of Genius (7th level): The artificer could use their reaction to add their Int mod to another creature's death saving throw. (They can't use it on their own death saves, since they're normally incapacitated while unconscious, which prevents taking reactions.)
  [Eberron: Rising from the Last War, p. 57; Wayfinder's Guide to Eberron, p. 179]

• Soul of Artifice (20th level): The artificer gains a bonus to all saving throws (including death saves) based on their number of attuned magic items.
  [E:RftLW, p. 58; WGtE, p. 180]

• Alchemist subclass - Experimental Elixir (3rd level): If the created elixir has the "Boldness" effect, it adds 1d4 to all of the drinker's saving throws (including death saves) within 1 minute. However, the effect of the elixir made for free at the end of a long rest is randomly determined; the artificer must spend a spell slot to make an additional elixir and choose its effect.
  [E:RftLW, p. 58; WGtE, p. 180]

Bard:

• Bardic Inspiration (1st level): The bard can grant Bardic Inspiration to another creature that can hear it; if that creature makes a death saving throw within the next 10 minutes, it could roll the Bardic Inspiration die and add the result to one death saving throw. (An creature can't gain Bardic Inspiration while unconscious, since it's unaware of its surroundings.)

Cleric:

• Peace Domain subclass - Emboldening Bond (1st level): The cleric can create a bond between a number of willing creatures equal to their proficiency bonus for 10 minutes; then, while any bonded creature is within 30 feet of another, it can add 1d4 to one death saving throw it makes (once per turn).
  [TCoE, p. 33]

Druid:

• Circle of Stars subclass - Cosmic Omen (6th level): When the druid finishes a long rest, they can roll a die and gain access to a special reaction that varies depending on whether the result is even or odd. If that die roll result is even, then the druid can use their reaction to add 1d6 to a death saving throw made by a creature they can see within 30 feet. (If the initial die roll result is odd, the druid can use their reaction to subtract 1d6 instead.)
  [TCoE, p. 39]

Fighter:

• Indomitable (9th level): The fighter could reroll a death saving throw that they fail, but must use the new roll.

Monk:

• Diamond Soul (14th level): The monk gains proficiency in "all saving throws" (which includes death saves). The monk could also spend 1 ki point to reroll a failed death save and take the second result.

• Way of the Drunken Master subclass - Drunkard's Luck (11th level): If the monk has disadvantage on the saving throw (e.g. from 3 levels of exhaustion), they can spend 2 ki points to cancel it for that roll.
  [Xanathar's Guide to Everything, p. 34]

Paladin:

• Aura of Protection (6th level): The paladin and all friendly creatures within 10 feet add the paladin's Cha mod to all saving throws (including death saves). The aura's radius increases to 30 feet at 18th level.
  (See this Q&A: Does a Paladin's Aura of Protection Affect Allies' Death Saving Throws?)

Ranger:

• Monster Slayer subclass - Supernatural Defense (7th level): The ranger adds 1d6 to every saving throw that the target of their Slayer's Prey feature "forces [them] to make".
  (How this feature interacts with death saves may warrant its own separate question.)
  [XGtE, p. 43]

Sorcerer:

• Divine Soul subclass - Favored by the Gods (1st level): The sorcerer could add 2d4 to a failed death save, possibly changing the outcome.
  [XGtE, p. 50]

• Wild Magic subclass - Bend Luck (6th level): The sorcerer could use their reaction and spend 2 sorcery points to add 2d4 to the death saving throw of another creature they can see.
  [Player's Handbook, p. 103]

• Clockwork Soul subclass - Trance of Order (14th level): The sorcerer can use a bonus action to enter a trance state for 1 minute; for the duration, whenever the sorcerer makes a death save, they can treat a roll of 9 or lower on the d20 as a 10. (As a result, it's impossible to fail a death save while in this state, barring other negative modifiers to the death save.)

Warlock:

• The Fiend subclass - Dark One's Own Luck (6th level): The warlock could add 1d10 to their own death saving throw.

• Protection of the Talisman invocation (prerequisite: 7th level, Pact of the Talisman): When the wearer of the warlock's talisman fails a death save, they can add 1d4 to the roll, potentially making it a success.

Wizard:

• Chronurgy Magic subclass - Chronal Shift (2nd level): The wizard could use a reaction to make another creature it can see reroll its death save and take the second result.
  [Explorer's Guide to Wildemount, p. 184]

• Chronurgy Magic subclass - Convergent Future (14th level): When another creature the wizard can see makes a death save, the wizard could use a reaction to ignore the die roll and decide that the number rolled is the minimum needed to succeed (i.e. a 10, if there are no other modifiers).
  [EGtW, p. 185]

• School of Divination subclass - Portent (2nd level): Before the wizard or a creature they can see makes a death save, the wizard can replace the save with one of their foretelling rolls.
  [PH, p. 116]

Feats

• Bountiful Luck: The character could use their reaction to let an ally reroll a natural 1 on a death saving throw (they must use the new roll).
  [XGtE, p. 73-74]

• Lucky: When making a death saving throw, the character could spend a luck point to roll an additional d20, then choose which one is used for the death save.
  [PH, p. 167]

Spells

• Bless (1st-level): Targets can add 1d4 to all death saving throws they make for the duration. (It's a concentration spell, so it can't benefit the caster's death saves.)

• Ceremony (1st-level): A humanoid targeted by the "Dedication" rite could add 1d4 to all death saving throws they make for the next 24 hours. (The target can only gain this benefit once.)
  [XGtE, p. 151]

• Resistance (cantrip): The target can add 1d4 to a saving throw of its choice (including a death save) once before the spell ends. (It's a concentration spell, so it can't benefit the caster's death save.)

• Warding bond (2nd-level): The target (another creature) gains a +1 bonus to saving throws (including death saves) while within 60 feet of the caster. The spell ends if the caster drops to 0 HP.

Magic Items

• Astromancy archive (attunement by a wizard): The archive has 3 charges. When a creature the wizard can see within 30 feet makes a death save, the wizard can expend 1 charge while holding the archive and use their reaction to make the creature add 1d4 to the original roll. They can wait to do so until after they see the roll but before its effects are applied. (TCoE, p. 120)

• Ring of protection (attunement): The wearer gains a +1 bonus to saving throws (including death saves).

• Cloak of protection (attunement): The wearer gains a +1 bonus to saving throws (including death saves).

• Luck blade (attunement): A character gains a +1 bonus to saving throws (including death saves) while the sword is on their person.

• Robe of stars (attunement): The wearer gains a +1 bonus to saving throws (including death saves).

• Stone of good luck (luckstone) (attunement): A character gains a +1 bonus to saving throws (including death saves) while the stone is on their person.

Racial Traits

• Lucky (halfling): A halfling that rolls a 1 on a death saving throw may reroll it, and must take the second result.

Supernatural Gifts (Mythic Odysseys of Theros)

• Pious (MOoT, p. 17): The Pious Protection trait lets a Pious character who fails a saving throw reroll the save, but they must use the new roll.