A BASH function that should be built-in.
The create directory command, cd, is one of those commands that I use almost every day on the command line. I realized after listening to a recent podcast [footnote]Pretty much any Back to Work episode where they talk about TextExpander :)[/footnote], I should look at how many keystrokes I do and where I can make things more efficient.
[bash]function mcd() { mkdir $1 cd $1 }[/bash]