Randomize Transforms
Randomize Transforms is an editor extension that lets you quickly randomize the rotation, scale, or position of any number of objects in your scene at once. This easy to use tool is ideal for randomizing mass-placed scenery objects (such as rocks, trees, small plants, etc) to give your scene a more natural, varied, and visually pleasing look.
Asset Store link: Randomize Transforms | Utilities Tools | Unity Asset Store
To use, just open the Randomize Transforms window (Window > Randomize Transforms), select the objects you want to randomize, choose how you want to randomize them, and click ‘Apply’.
Selection of objects is done by either search (name or tag) or manual selection of individual transforms. Search based selection allows you to select, for example, everything in the scene that has “rock” in it’s name at once.
Each type of randomization (rotation, scale, position) can be adjusted to suit a wide variety of use cases.
Randomizations can be applied to each axis individually, if desired.
For rotation and scale, you can choose to randomize objects to absolutes (example: set their scale to a random value between 0.5 to 3.0) or modify their current values (example: multiply their current scale by a random value between 0.9 to 1.1). This gives you the option of randomizing things dramatically, or just giving them some slight variation.