!$

There are plenty of posts around describing things that can be done at the command line with Bash history, but the first step into a smörgåsbord of functionality is almost always through a single convenient doorway. This doorway is !$, which Bash expands to the last argument of the previous issued command.

How often do you edit a file and commit changes?

$ vi data/science/project.py
# Make some changes
$ git add !$
$ git commit -m 'What a nice change that was!'

Or how about moving a file before editing it?

$ mv some/long/path/to/a/file.mdown some/long/path/to/a/file.md
$ vi !$

Though it looks weird onscreen, !$ is actually kinda enjoyable to type as well; it just feels good to fork the '1' and '4' keys while holding shift with the right pinkie. It's a straightforward and memorable gesture that the strange '!$' combination belies.

TL;DR: 10/10, would use again.