Image 1: Guitar, (Worm Gear)
The worm gear is used to tune the guitar. It reduces the output speed (which is the guitar strings), so tuning of the guitar can be more precise.
Image 2: Correction tape, (Gears)
The gears are used to slide the correction tape in and out, so it would take less space and the tape would not be dirtied since it is slided in.
Image 3: Tongs, (Class 3 Lever)
The lever reduces the effort needed to pick up things using the tongs. It also make it easier to pick it up.
Image 4: Bookshelf door (Linkage)
The linkage limits the movement of the door. This linkage enables the door to open up till 90 degrees.