Shamelessly copied from this tiny twitter thread by Kristian Köhntopp

$ !keks

Search history for the last command starting with keks

$ !?keks

Search history for the last command containing keks

$ ^keks^cookie

Repeat the previous command, after replacing keks with cookie

$ !! 

Repeat the previous command

$ !!:s/image/headers/

Repeat the previous command, after substitution of image with headers

$ !!:gs/image/headers/

Repeat the previous command, after substitution of image with headers on the whole command line

$ sudo !!

Repeat the previous command but prefix it with sudo (so much yelling!)

$ shopt -s histverify

Show result of above history expansion before exec.

$  rm -rf keks

Starting a command with a space will prevent it from entering the history.

$ unset HISTFILE

unsetting HISTFILE or HISTFILESIZE will prevent the shell from logging commands

And of course, as mentioned later in that discussion: . inserts the last used word of a shell cmdline.