The farms that Rendog did are amazing (Hermitcraft S5). I loved his item elevator idea!
Making mob farms are probably the hardest, but most satisfying, farm to make (not including the redstone parts of transporting them, etc.). Automatic mob farms can either use lava, cactus, or even water drowning, but I would suggest using either fall damage (and then an item elevator) or a Vindicator named "Johnny." This causes the Vindicator to kill ANY nearby mobs.
Maybe having the produce flow underground (having glass to view it) is the cheapest way to make an item transporter and one of the most satisfying.
Limo!