Is there a quick way to increase/decrease a cell value by 1 in Apple iWork's Numbers?
Numbers provides an interface for AppleScript. Open Automator.app and create a new Service. Here, drag Run AppleScript from the left to the right pane.
Now, paste the following AppleScript with a bit of copypasta from here:
Save this workflow as Increase Cell Value or similar. Close the service and create another one. Now, do the same again, but change
Finally, go to System Preferences » Keyboard » Keyboard Shortcuts. Here, under Services, assign a keyboard shortcut to your new actions, for example ⇧⌘I to increase and ⌥⇧⌘I to decrease.
Once this is done, you can just select any number of (numeric) cells in Numbers and hit your (global) keyboard shortcut.
Before and after:
Your services are also available through the Numbers menu:
Of course, one could do some further error checking for cells that contain text, but you get the idea.