FavorMe

Works with Unity 4.5 and up, including Unity 5.
An extremely simple and lightweight favoriting tool designed for speed and simplicity.
You have the option of adding up to 3 stars to any game object in the hierarchy, simply by clicking the star icon next to the desired hierarchy object, and with a right click you can quickly remove all stars.
An easy to reach button at the top of the hierarchy menu allows the hierarchy to be sorted by star amount. Simply select the FavorMe sorting option, and the more stars an object has, the higher in the list it will be, it's as simple as that.
v1.1 - Added undo functionality. It uses Unity's built in undo feature, so just press Ctrl-Z to undo.