The computer keyboard will probably be with us for many years to come – it’s just such an efficient way (once you get used to it) of text entry, that it’s hard to imagine it’ll be replaced entirely with gestures or by speech.
There are some pretty obscure keys on the standard PC keyboard though – many of which date to the very earliest implementations of the IBM PC. What does Scroll Lock do, for example, other than annoy Excel users who think they’re moving the cursor around inside the sheet, only to find the whole thing is scrolling up and down?
The Pause key (often doubled up with Break, which dates back to the days of the telegraph) has one interesting modern side effect – press WindowsKey + Pause, and your machine will jump straight to the “System Properties” page – a handy way of checking the config of a machine you’re using.
These kinds of tips were once redolent of the doyen of desk-side PC support, where every second spared in visiting a user was time better spent in the pub. All of this is of course lost now, what with the risk in desktop sharing via Lync or Remote Desktop software.
The AltGr key normally found to the right of the space bar has a few odd functions that are not often needed, from a way of setting formatting in Office to a means of entering accented characters. Try AltGr+e for example to chuck an é into a name, and keep people with extravagant names happy that you’ve bothered to spell them correctly. There are other ways of doing the same thing, too – Office apps all have a means of using “dead keys”, eg CTRL+ ‘ followed by an appropriate letter would render an acute accent, or the CTRL+ ` (generally found on the key below Escape) will render the next letter with a grave. CTRL + Caret (^), Colon (:) or Tilde (~) will accent the following letter with the appropriate accent. See here for more international Office fun.
Finally, there’s the strange “menu” key, sometimes referred to a “application” or “right click” – usually found to the right of AltGr. It’s generally used as the equivalent of right-clicking a 
mouse, though can be followed up with other keys to quickly perform functions that might otherwise need a few clicks or menu commands.
One example – if you are looking to paste some text in a document or email, you can quickly press the menu button then follow with T if you only want to paste the text only (ie plain text, not the formatting) or M if you want to merge formatting.
In a Metro Modern application, the menu/application/right-click key also has the same effect as swiping up from the bottom of the screen (or pressing WindowsKey + Z).
