Inspector Gadgets Lite is Free, but only has a reduced set of features compared to Inspector Gadgets Pro.

The full Source Code is included in Inspector Gadgets Pro as regular C# scripts. The InspectorGadgets.Lite.dlls in the package contain no code, they are simply there to allow seamless upgrades from the Lite version.
Customise settings in the Edit/Preferences window.
Enhanced Transform Inspector
Toggle between Local and World values.
Freeze Children so that they won't be affected when you move the parent.
Draw All Gizmos so that you can move each selected object individually.
Uniform Scale draws a single field to scale the object on all axes at once.
Buttons to quickly Copy, Paste, and Snap values.
Middle Click on any field to Reset it.
Movement Guides display the starting position and distance moved when moving objects in the Scene view.
Works with multiple objects selected at once.
Context Menu Functions
Various Context Menu Functions are added to all Inspector fields depending on their type. Functions include Copy, Paste, Find, Visualise, and Watch.
Ctrl + Alt + I keyboard shortcut to open a new Inspector window locked to the current selection.
Persist After Play Mode functions on Components and Inspector fields allow you to avoid losing changes you made during Play Mode once you return to Edit Mode.
Script Inspector
Improved Object Reference Fields can draw the referenced object's Inspector below the field as well as a button to quickly find an object of the right type.
The Script Field normally shown at the top of each Component's Inspector is hidden by default. You can Middle Click to open the script and if you show the field you can change the script's type without removing it and adding a new one.
Decorator Attributes allow you to easily customise the appearance of the Inspector without needing to write a custom Inspector class.
Inspectable Attributes allow you to add extra elements at the bottom of the Inspector.
Custom Event Messages allow you to replace or add to the Inspector without needing to write a custom Inspector class.
Easily find and fix Missing Script references.
Other Features
Automatically Hide UI objects when you aren't working with them.
Drag and drop assets into each other to turn them into sub-assets.
Leave Comments as components and assets.
AutoPrefs make it easy to work with PlayerPrefs and EditorPrefs.