For more than 20 years, NetBSD has shipped X11 with the “classic” default window manager of twm. However, it’s been showing its age for a long time now.
In 2015, ctwm was imported, but after that no progress was made. ctwm is a fork of twm with some extra features – the primary advantages are that it’s still incredibly lightweight, but highly configurable, and has support for virtual desktops, as well as a NetBSD-compatible license and ongoing development. Thanks to its configuration options, we can provide a default experience that’s much more usable to people experienced with other operating systems.
The ctwm website has more information for those interested.