Cacti have always been farmable completely automatically. With pistons in 1.7, sugar cane can now be farmed in a fully automatic fashion as well. Mobs and animals can be farmed as well in giant mob grinders. To my knowledge these are the only items that can be farmed in a fully automatic fashion in vanilla Minecraft.
Cobblestone can be farmed fully automatically using mods, or in a semi-automatic fashion using TNT. Wheat can be farmed semi-automatically on a massive scale, but unfortunately requires replanting and tilling.
What I'd like to know is if there are any contraptions people have contrived to harvest any other item in a fully automatic way without using any mods.