A set of programs to change file and directory names by editing them inplace; I find imv especially useful to edit a filename at the program prompt.
Mail client with tons of features, customization chances, support for IMAP, POP3, multiple storage formats.
peco is a CLI utility that filters text interactively. The tool is written in the Go programming language. It's free and open source software.
As used to author Heaven's Vault, 80 Days and Sorcery!: produce interactive scripts by writing in pure-text with ink markup.
Ascii Patrol is an ASCII game project. It was mainly inspired by "Moon Patrol", my favourite arcade game at the times I was a child.
Terminal utility that allows you to use words coming from the standard input to create a nice selection window just below the cursor. Once done, your selection will be sent to standard output.
percol is an interactive grep tool in your terminal. percol
Since percol just filters the input and output the result to stdout, it can be used in command-chains with | in your shell (UNIX philosophy!).