Winkey+D to minimise all windows, then Winkey+D to restore them all.
Winkey+L to quickly lock my work machine
 

Alt+S to send an email in Outlook.

Found this useful whilst currently working from home over a laggy RDP connection to my work desktop PC, where every mouse move is slow. The less mouse the better.

Ctrl-Shift-Delete

Clear recent history (cache preselected). Maybe not my "favourite" since I only discovered it yesterday while doing frequent updates of cached JSON bits.