Grass

Tall Grass was implemented in the game in Beta 1.6. . It spawns on Grass blocks in certain biomes. Seeds are no longer able to be produced from grass blocks and only spawn from harvesting these blocks. Little else is known about it, however. Tall grass isn't a solid block, but rather an aesthetic weed that does not block movements, like Sugar Cane, Shrubs and Wheat; just occurring naturally in large quantities.

Trivia

 * The color of tall grass is biome dependent, and the same color as grass.
 * Tall Grass does not drop when destroyed, therefore it's impossible to replant it.
 * Tall Grass is also offset from its block as can be seen in the third screenshot.(This seems intentional since it's a random offset and also has a height offset.)
 * If placed on normal grass, tall grass blocks appear as Dead Shrubs.
 * It is currently possible to place blocks on top of tall grass, although said grass will (in most cases) get destroyed afterwards.