I started building a farm on my survival server. It's turned out pretty nicely so far. The cactus farm generates about 20 blocks of cactus per minute, the wheat farm can be used to gather 2,400 wheat with one button click and the dye farm is really helpful, too.

I initially had some trouble with getting the water to cover the entirety of the wheat farm, since water physics in Minecraft are a bit weird, but that didn't take long to solve. The only problems with the design I'm using now are that water sources are created at the top, and require a second button press to be removed, and that the quantity of wheat floating downstream can cause your frame rate to drop quite severely.

The mushroom farm is now finished. It's located underneath the farmhouse; there's a trap door in the corner of the house, with a light switch right next to it. The mushroom farm is at light level 0 when the lights are off, and is bright enough when they're on to stop mobs from spawning. It can produce 1,600 mushrooms at a time (800 of each type). Since it's so dark with the lights off, a lot of mobs spawn. I'd advise turning the lights on, walking a few hundred metres from it, so the mobs despawn, and returning to gather the mushrooms.

I also built a 64x64 block field for passive mob farming. I didn't see much point in an automatic passive mob trap, since you can get wool far more quickly by using shears. I built the field in a lake, so that there isn't any other grassland close by and so mobs are more likely to spawn in the field during the day.

The farmhouse is almost complete. I'll probably build a cocoa farm in the attic if and when that becomes possible.

There are still a few things I have planned for the farm:
- build a sugarcane farm;
- turn all of the grass surrounding the wheat farm to dirt in order to prevent passive mobs from spawning and trampling the crops; and
- finish the farmhouse;
- build a watermelon farm in 1.8; and
- build a pumpkin farm in 1.8.

I hope you like it. If you have any suggestions for improvements or new buildings, feel free to post them in the comments.

Additional Notes

A giant farm used to gather wheat, cacti, seeds, red and yellow flowers, red and brown mushrooms, passive mob drops and soon sugar cane.