A quick overview of all the class/method/field attributes Unity has.

Components

• HelpURLAttribute set a custom URL for documentation.
• DisallowMultipleComponent prevents multiple instances of this component in a game object.
• RequireComponent this component requires another component to be present in the game object.
• SelectionBaseAttribute defines which object will be picked when selected in the scene view.

Component Fields

• HideInInspector hide the field from inspector.
• SerializeField serialize the field, makes non-public fields visible.
• TooltipAttribute set help text on field mouse-over.
• SpaceAttribute add spacing between fields.
• HeaderAttribute add heading text above fields.
• DelayedAttribute this field is not updated until user presses enter or removes focus.
• RangeAttribute restrict int/float to be in the specified range.
• MultilineAttribute change string field to multiline textbox.
• TextAreaAttribute change string field to flexible text area.
• ColorUsageAttribute configure color field's color picker settings.

Menus

• AddComponentMenu add custom script in specified Add Component menu.
• ContextMenu add this method in context menu of the component.
• ContextMenuItemAttribute add a method under context menu of this field.
• CreateAssetMenuAttribute add this scriptable object under Assets/Create.
• MenuItem add a method to the editor main menu.
• PreferenceItem add preference section to Preferences window.

Rendering

• ImageEffectOpaque make image effect to be rendered after opaque geometry.
• ImageEffectAllowedInSceneView make image effect to be rendered into scene view camera
• ImageEffectTransformsToLDR switch image effect to LDR rendering.

Runtime

• RuntimeInitializeOnLoadMethodAttribute automatically call a static function after a runtime scene has been loaded.
• SharedBetweenAnimatorsAttribute makes that this StateMachineBehaviour is only initialized once and shared between all animators.

Editor

• InitializeOnLoadAttribute automatically initialize a class after editor launches.
• InitializeOnLoadMethodAttribute automatically call a static function after editor launches.
• DidReloadScripts automatically call a static function after scripts have been reloaded.
• ExecuteInEditMode make a script execute even when not in play mode.
• DrawGizmoAttribute add gizmo icon for this component type.
• GUITargetAttribute controls to which display is OnGUI UI rendered.
• OnOpenAssetAttribute allow overriding what Unity does when you open an asset.

Custom Editors

• CustomPropertyDrawer specifies which runtime class this drawer is for.
• CustomEditor specifies which runtime class this custom editor is for.
• CanEditMultipleObjects allows multi-object editing for this custom editor.
• CustomPreviewAttribute allow adding custom preview for containing game object.

Building

• PreserveAttribute prevents a class, method or field from being stripped if build code stripping is enabled.
• PostProcessSceneAttribute automatically call a static function after each scene build.
• PostProcessBuildAttribute automatically call a static function after whole build.

Sources

• Unity Docs - Attributes
• Unity Attributes