Everyone assumes that using keyboard shortcuts is more efficient than using the mouse. But by how much? One 2005 study found that keyboard shortcuts are approximately twice as fast.
So why don't more people use them? It's apparently not an experience issue. A separate study found that keyboard shortcut usage does not tend to increase over time, but rather correlates to social experiences. People learn to use shortcuts from watching others.
With that in mind, here is a cheat sheet of my personal most commonly used keyboard shortcuts. I'm not making any claims to exhaustiveness; in fact I intentionally left out commonly used but also commonly known shortcuts like "Alt + Tab".
I'm also not claiming that these are generally applicable. I'm a developer, so I included Eclipse, etc. Others are Linux specific, which automatically rules out 99% of users. Others require customisation to enable.