Before its official release, Minecraft 1.8 was "leaked" for testing. Today, a pre-release version of 1.9 was similarly "leaked." I guess this is becoming a trend. 1.9 is another part of the Adventure Update, which has now been split again, with 1.10 coming later with even more adventure changes.

1.9 includes many interesting new features. NPCs now populate villages (though they currently do nothing), the snow biome is back (and red flowers are too), and swamps were made...swampier, with tinted water, lily pads, and other changes. There are new giant mushroom biomes with Mooshrooms (not a joke), new Nether fortresses made of new blocks and with new enemies, and you can even create "snow golems" (snowmen) with some snow and a pumpkin. Also, lava and water now visibly "leak" (particle effects) through blocks below them.

As this leak is a pre-release, there are of course some issues. Lighting issues are not fixed entirely, swamp water transitions look strange, and leftover debug code makes jumping change your experience. Also, holding down space no longer allows you to continuously jump, but Notch has already pledged to change this back to normal tomorrow.

If you can't wait to try out the new update, grab the pre-release. An updated server.jar is available too. Otherwise, wait for the official release.