Features:

• Unselectable objects: click the hand on any Hierarchy row and that object, with everything under it, can't be picked in the scene view. Clicks go through it to whatever's behind, and a drag that starts on it works as if on empty space.
• The Hierarchy and the inspector can still select an unselectable object. It's saved with the scene, takes undo, and counts as an override on prefab instances.
• Alt+L toggles the selected objects. Scene > View > Make All Selectable frees everything, and right-clicking the scene view frees the object under the cursor.
• Drag selection that takes only what's fully inside the box: the small things you drew around, not the group or the map they sit in. Sprites count, prefabs are taken whole, and Shift adds and Ctrl removes as usual.
• The Hierarchy shows what a drag will select while you're still dragging.
• A selection outline: a box around each selected object and everything under it, so a selected prefab or group shows up in the scene view.
• A bounding box around the whole selection, marked by its corners, like in PlayCanvas.
• In 3D, the lines can hide behind objects, or show through them at an opacity you choose.
• Settings in Edit > Preferences > Hierarchy Extension: the outline and the bounding box on or off separately for the 2D views and the 3D view, plus line thickness and colour.
• Editor only: nothing is added to your game, and there are no components to place.

How to:

1. Install it from View > Library Manager: search for Hierarchy Extension on the Browse tab and press Install.
2. In the Hierarchy, hover a row and click the faint hand at its right-hand end, next to the eye. It turns into a crossed hand, and the object can't be selected in the scene view any more.
3. Click where that object is in the scene view. The click goes through to whatever is behind it.
4. Drag a box around a few things. Only what's fully inside gets selected, and each selected object is outlined.
5. To make an object selectable again, click its crossed hand, press Alt+L, or use Scene > View > Make All Selectable.
6. To change the outline, open Edit > Preferences > Hierarchy Extension. The 3D outline and the 3D bounding box are off until you turn them on.

Settings:

• Outline in 2D Views - [Default: on]: a box around each selected object in the Top, Front, Side and Flat 2D views. 
• Bounding Box in 2D Views - [Default: on]: the corners of one box around the whole selection in the 2D views.
• Outline in 3D View  - [Default: off]: a box around each selected object in the 3D view.
• Bounding Box in 3D View  - [Default: off]: the corners of one box around the whole selection in the 3D view.
• Hide Behind Objects in 3D - [Default: on]: objects in front hide the lines. Turn it off to see the lines through everything.
• Hidden Line Opacity - [Default: 0.5]: with Hide Behind Objects in 3D off, how strongly a line shows where something is in front of it, from 0 (not at all) to 1 (fully).
• Line Thickness - [Default: 1.5]: how thick the lines are, in pixels, from 0.5 to 6.
• Line Color - [Default: White]: the colour of the lines, the editor's own selection colour to begin with.