F3

F3 is useful for whenever you need to free your mouse. You can for example press F3 then hold C afterwards and then you can toggle third person on and off. 

F3 also displays in details of how many server wide events (such as printer event, gambling event, crafting event etc.) are currently running and which ones.

Once you press F3 you need to press it again to go back to normal mode.

If you have enabled Profit/Loss tracking in F4 settings, pressing F3 will also open a popup in the top left of your screen with your money stats.