Well, I've recently been playing on a server with no protection system for normal players outside of chests etc. and so I came up with this door to keep my precious crops safe. It's simple and fast to build, and will probably keep out most unaware intruders unless they mine through the wall. It uses a redstone torch to open the door in my build, but you could also use a lever, pressure pad, or button or really anything that turns off the torch powering the door, but I think it's simplest to just use the torch. Also, you must note that the door does not close behind you.

The Inner Mechanism

See the torch above? That powers both pistons. It MUST be attached to the block like it is in the picture, or else it won't work, because the space under that block is where the second redstone torch goes, turning off the torch and opening the door.