Ask HN: What are the useful, lesser known keyboard shortcuts?

I'm a Mac user, and suspect I only use six or seven keyboard shortcuts regularly (close window, quit, new, application switch, save, copy/paste, etc).

I suspect we find it difficult to learn TOO many (i.e. all) shortcuts, so which are the lesser-known ones (on whichever operating system) that you think people should know?

For example, I just learned the incredibly useful Apple-` shortcut to switch between Windows (not Apps), e.g. for working on multiple documents.

Any suggestions?