Is there a way to make all layers visible in Photoshop? Is there a quick way to do this, for example, short key or script?
I'm guessing you have a huge number of layers:
The fastest way is to click+drag over all the layer eyeballs, this works in CS5 at least. Note that visibility changes are not undo-able, i.e. they don't show up in history
A better suggestion for the future; get used to organizing layers into groups, then you will have far less 'points' to turn on/off layers.
There are also some cool layer shortcuts here
Show/Hide all layers:
You can use the "show all/hide all layers" by right clicking on the eyeball on any layer and selecting the "show/hide" option. It will make all the layers visible.
You can also use the other shortcut mentioned above (Command + Alt + A) and then use the shortcut Command + ,
Show/Hide all layers but one:
If you want to hide all the other layers but not a specific one, you can hold the ALT key and click on the eyeball on the selected layer. It will show/hide all the active layers but the one you selected.
It will not make the layers that were not visible.... visible, though!
Show/Hide one layer
Simply click Command + , (it does the same as clicking the eyeball on one layer).
You can change that shortcut in the menu Edit > Keyboard Shortcut > Layers