Splash Potion

Splash Potions are future craftable throwable potion based weapons introduced in 1.9 Pre-release 4. You can turn any brewable potion into a Splash Potion by mixing in Gunpowder. It appears in a different shaped flask. Instead of drinking it, you throw it like a snowball or egg. Splash potions can also be fired from a Dispenser, and affect both passive and hostile mobs (including instant health and regeneration potions.) They give a use for negative potions, allowing you to use them against enemies. They can also be used to help allies (wolves or other players in SMP) when used with a positive potion.

Brewing
For ingredients that go into Potions, Splash Potions, and known Potion recipes see Brewing. According to Jeb there are 161 possible potion combinations in Beta 1.9 Pre-release 3, and there may be 2,653 potions in the full Beta 1.9 release.

Mob Immunity
Certain Mobs are immune to damaging splash potions such as Poison and Instant Harm.

Immunities needing further confirmation are in italics.

* It would be logical that since Ghasts (and Skeletons) used ranged attacks, that they would not be hindered by Weakness since it only affects a mob's melee attack.

Trivia

 * Effects from Splash Potions travel roughly five blocks in all directions from the source of impact.
 * A splash potion will affect the player who throws it if it impacts close enough. This can be dangerous if using harmful potions but can also be useful; a group of players clustered together could all receive the benefit from a positive potion, depending on distance, if the user throws it straight down at their own feet.
 * A Splash Potion's effect will decrease in time the further away the target is from the explosion centre.
 * As of its first appearance, Splash Potions do minor explosive push damage and have an in-game damage effect, but they do not seem to harm Creepers or Skeletons in Creative Mode.
 * On reddit jeb said that water splash potions that have no effect could be used against endermen to deal damage