Dwarves have advantage on saving throws against poison.
Elves have advantage on saving throws against being charmed.
Halflings have advantage on saving throws against being frightened.
Stout halflings have advantage on saving throws against poison.
Fighters' protection stance can use a reaction to give disadvantage to an attack made against a creature less than 5 feet away.
Rogues of level 18+ negate the advantage of any attack against them as long as they are not incapacitated.
Thief rogues of level 9+ have advantage on Dexterity (Stealth) checks if they move at no more than half their speed that turn.
Wearing an armor you are not proficient with causes disadvantage on any ability check, saving throw or attack roll that involves Strength or Dexterity (plus you can't cast spells).
Wearing some armors causes disadvantage on Dexterity (Stealth) checks.
Small creatures have disadvantage on attack rolls with heavy weapons.
Lances attack at a disadvantage when the target is within 5 feet of you.
Drinking an antitoxin gives advantage on saving throws against poison for 1 hour.
Using a crowbar grants advantage to Strength checks when it can be applied as leverage.
A magnifying glass grants advantage on any ability check to appraise or inspect a small or detailed item.
When another character helps you use a ram to break a door, you gain advantage on the Strength check.
Saddles on a mount give advantage on any check to remain mounted.
Using the Dodge action during combat has any attack roll against you made at a disadvantage until the start of your next turn (if you can see the attacker). Dexterity saving throws are made with advantage.
Using the Help action during combat can give an ally advantage in one of his/her own ability checks before the start of your next turn (see "working together"). Alternatively, it can provide advantage on the first of an ally's attack rolls against a monster.
Attacking an enemy while hidden (if they don't detect you approaching) or otherwise unseen grants you advantage on attack rolls. Conversely, attacking an enemy you can't see has you making them with disadvantage.
Ranged attacks whose target is within a weapon's long range (but not within normal range) have a disadvantage on the attack roll.
Ranged attacks in close combat (within 5 feet of a hostile creature who can see you and isn't incapacitated) have a disadvantage on the attack roll.
In a lightly obscured area, Wisdom (Perception) checks that rely on sight are made at a disadvantage.
When making a melee weapon attack underwater, a creature that doesn’t have a swimming speed (either natural or granted by magic) has disadvantage on the attack roll unless the weapon is a dagger, javelin, shortsword, spear, or trident. Ranged attacks against targets within normal range can be made, but with disadvantage on attack rolls.
Beacon of Hope causes targets to have advantage on Wisdom saving throws and death saving throws.
Blur causes creatures to have disadvantage on attack rolls against you (if they rely on sight).
The targets of Charm Person, Dominate Person, and Dominate Monster do their Wisdom saving throws with advantage if you and/or someone friendly to you is fighting them.
Wisdom saving throws against ill effects of Dream are made at a disadvantage if the caster has a portion of the target's body.
Attacks rolls against targets affected by Faerie Fire are made with advantage if the attacker can see them.
The target of Foresight has advantage on attack rolls, ability checks, and saving throws. Attack rolls against him/her are made at a disadvantage.
The next attack roll made against a target hit by Guiding Bolt (before the beginning of the caster's next turn) has advantage.
Someone who consumes the food and drinks generated by Heroes' Feast makes all Wisdom saving throws with advantage for the next 24 hours.
Creatures within the radius of a Holy Aura (who are chosen by the caster) have advantage on all saving throws, and other creatures have disadvantage on attack rolls against them.
The target of a Haste spell has advantage on Dexterity saving throws.
The target of Otto's Irresistible Dance has disadvantage on Dexterity saving throws and attack rolls. Other creatures have advantage on attack rolls against him/her.
Casting Resurrection on a creature that has been dead for more than a year causes the caster to have disadvantage on all attack rolls, ability checks, and saving throws until he/she finishes a long rest.
The melee spell attack granted by Shocking Grasp is made with advantage if the target is wearing a metallic armor.
Undead and oozes have disadvantage on the saving throw against the Sunburst spell.
Attack rolls against a blinded target have advantage.
Attack rolls made by a blinded creature have disadvantage.
Attack rolls against an invisible target have disadvantage.
Attack rolls made by an invisible creature have advantage.
Attack rolls against a paralyzed target have advantage.
Attack rolls against a petrified target have advantage.
Attack rolls and ability checks made by a poisoned creature have disadvantage.
Attack rolls against a prone target have advantage if the attacker is within 5 feet, or disadvantage otherwise.
Attack rolls made by a prone creature have disadvantage.
Attack rolls against a restrained target have advantage.
Attack rolls and Dexterity saving throws made by a restrained creature have disadvantage.
Attack rolls against a stunned target have advantage.
Attack rolls against an unconscious target have advantage.
The charmer has advantage on any ability check to interact socially with a charmed creature.
A frightened creature has disadvantage on ability checks and attack rolls while the source of its fear is within line of sight.
Level 1 of exhaustion causes disadvantage on ability checks.
Level 3 of exhaustion causes disadvantage on attack rolls and saving throws.
Inspiration: given by the dungeon master, can be spent to gain advantage on an attack roll, saving throw, or ability check.
Working together: When two or more characters are working together to attempt a task, the one leading the effort is granted advantage on his/her check.
Optionally, the encumbrance rule can be used for lifting and carrying weight. Carrying weight in excess of 10 times your Strength score causes disadvantage on ability checks, attack rolls, and saving throws that use Strength, Dexterity, or Constitution.
When squeezing through a space one size smaller than itself, a creature has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against it are made at an advantage.
After three days of downtime between adventures spent recuperating from a series of diseases and/or poisons, on a successful DC 15 Constitution save, you gain advantage on saving throws against one of them for 24 hours.