A file manager, currently produced by GPSoftware. (http://www.gpsoft.com.au/).
Directory Opus became a somewhat legendary file manager on Amiga. It's not a mere file manager, but rather a complete replacement for the Workbench, and adds a bunch of cool stuff to it.
A version for Microsoft Windows also exists nowadays, and, naturally, DO has spawned numerous clones (for example, gentoo).
It's a commercial product - a version for Amiga comes with Amiga Forever (DO 5.5, which doesn't seem to like AmigaOS 3.9 colored icons...) Also, the source code for old version (4.12) of DO has been released under GPL.