Isolate
When you got a crowded Hierarchy but want to make changes to only to a small subset of GameObjects, Isolate can be your friend.
You may often find yourself under this situation when developing your UI where lots of nested GameObjects are needed. Wouldn't it be cool if you could deactivate all the other GameObjects you don't care about? Well, that's exactly what Isolate does for you!
Use it by selecting a GameObject you intend to work on, then press ctrl+e. It will deactivate all GameObjects except the parents and children of the one you selected.
In order to undo the isolate effets, you can simply press ctrl+e again.
Before
After
Alternatively, you can also activate it by clicking the "little gear" icon on the top right corner of the Transform component and then selecting Isolate.
It is not possible to isolate twice in a row, in order to isolate another group, you have to first undo the first isolate you made.
Also, it's recomended not to change scenes or recompile code before you undo the isolate changes. By doing so, you risk losing the previous scene state. ;)