curses and termcap: a hardware-independent library for controlling cursor movement, screen editing, and window creation on ASCII display terminals, based on termcap (based on Bill Joy's vi screen control code). Curses was a landmark display library that made it possible for a vast number of new applications to create full-screen user interfaces that were portable between different brands of display terminal.
fortune: a fortune cookie program. Although Arnold's quote-displaying program was not the first in history, as the BSD standard it became by far the most widely used, and its database of quotes was voluminous. It also standardized a plain-textfile format that was philosophically aligned with Unix and thus became widely used both for other fortune programs as well as non-fortune purposes.
Ctags: a very early special-purpose hypertext link generator that essentially turned the vi editor into an IDE. It indexed program objects (such as functions) so that a user of vi (or a clone such as vim) could navigate to an object or function definition from any instance of the object's name elsewhere in the source code.
Additionally, Arnold served as both a member of the student senate and as its president.