Redstone and Misc device ideas?

[Nuro]

Curious if anyone has any cool ideas for devices to build using redstone (and redstone accessories) and various other blocks (e.g.: water, ice). I think its pretty fun to build these sorts of things and it'd be cool to hear some ideas. Especially functional ones. :3

[RockCambo]

Automated Turret Guns. Because why not. Or Make a big Redstone cannon that shoots TNT into the water because as we all no water can't feel the pain of explosions.

[SakanaSama]

you can jury rig a 2x2 hidden door together in the matter of a half hour. And there is the ever popular Auto-farm