unix tools, vidir
I'm a fan of the unix tools philosophy, but I sometimes wonder if there's much room for new tools to be added to that toolbox. New programs that fully partake of the philosophy (like rename, netcat, par, bloxsom, grepmail, and xwit) are fairly few and far between compared to all the new programs that don't. I've always wanted to come up with my own general-purpose new unix tool.
vidir doesn't quite qualify as a real unix tool, since it doesn't (normally) operate on stdio, but it's as close as I've gotten, and it does seem to fill in a nice hole in the toolbox. The idea is very simple, it just displays the files that are in a directory as lines in a text editor, then you can make any changes you like, and when you quit the editor it renames and/or deletes files to match your changes. No need to worry about quoting issues and the full ad-hoc editing power of a good text editor make cleaning up messy directories fun.
Like any good utility, there is plenty of obvious room for improvement, it would be nice if it had a --recursive switch to edit whole directory trees at a time. And it needs to have some support added for swapping pairs of filenames.
I challange others to try to make general purpose unix tools..