Just a list of 20 (now 28) tools for the command line. Some are little-known, some are just too useful to miss, some are pure obscure -- I hope you find something useful that you weren't aware of yet! Use your operating system's package manager to install most of them. (Thanks for the tips, everybody!)
Hotwire is an object-oriented hypershell. It is a shell designed for systems programming (files, processes), and thus it is in the same conceptual category of software as the Unix shell+terminal and Windows PowerShell.