F3.
I use it all the time but what happens is not always what you expect.
If you're lucky it searches or it does nothing.
F5 refreshes the web page in all graphical browsers, which is handy.
F2 renames a file (in Windows, anyway).
I used to tell people about F1 as well, which has meant "help" since the DOS days.
Unfortunately there's a trend among PC and keyboard manufacturers lately to replace the F keys with customized crap like volume controls, launching the browser or email client or putting the computer to sleep. This has been around for years, but lately Dell, HP and even Lenovo have taken to making this crap the default function of the F keys and requiring the user to use a "Fn" key to achieve the normal function rather than vice versa, particularly on laptops.