Can I hide/show objects by layer or tags?

Kind of 3DSMax-style.

For the editor view, uncheck the le layer you want to hide on the top left of the screen, where there is two list button "Layer" and "Layout". For the game view, take a look at your active camera, parameter "Culling mask".

Good luck

Note that the two “list buttons” mentioned seem to be in the top RIGHT corner of the editor now (in Unity 4.x). Don’t know whether this has changed in one of the newer versions, or whether it is just caused by my individual layout.

Unity 2018, “Layers” button in the top right hand corner. Click the eye icon to make a layer visible or invisible.

In Unity 6 it’s moved to a button inside of the Scene Window, in an overlay menu, and is only visible if the “View Options” button is toggled on first