Question: What familiar device was invented by human-computer interface engineer Douglas Engelbart, creator of hypertext?

Context paragraphs:
NLS (computer system): NLS, or the "oN-Line System", was a revolutionary computer collaboration system from the 1960s.  Designed by Douglas Engelbart and implemented by researchers at the Augmentation Research Center (ARC) at the Stanford Research Institute (SRI), the NLS system was the first to employ the practical use of hypertext links, the mouse, raster-scan video monitors, information organized by relevance, screen windowing, presentation programs, and other modern computing concepts.  It was funded by the Defense Advanced Research Projects Agency, NASA, and the U.S. Air Force.

Douglas Engelbart: Douglas Carl Engelbart (January 30, 1925 – July 2, 2013) was an American engineer and inventor, and an early computer and Internet pioneer.  He is best known for his work on founding the field of human–computer interaction, particularly while at his Augmentation Research Center Lab in SRI International, which resulted in creation of the computer mouse, and the development of hypertext, networked computers, and precursors to graphical user interfaces.  These were demonstrated at The Mother of All Demos in 1968.  Engelbart's Law, the observation that the intrinsic rate of human performance is exponential, is named after him.

Augmentation Research Center: SRI International's Augmentation Research Center (ARC) was founded in the 1960s by electrical engineer Douglas Engelbart to develop and experiment with new tools and techniques for collaboration and information processing.

The Doug Engelbart Institute: The Doug Engelbart Institute, previously known as The Bootstrap Alliance, is a collaborative organization founded in 1988 by the late Douglas Engelbart and his daughter Christina Engelbart, to research into the enhancement of human ability to solve complex, urgent problems.  Engelbart believed that it is possible to enhance society's collective intelligence by applying his strategies.

Questionnaire for User Interaction Satisfaction: The Questionnaire For User Interaction Satisfaction (QUIS) is a tool developed to assess users' subjective satisfaction with specific aspects of the human-computer interface.  It was developed in 1987 by a multi-disciplinary team of researchers at the University of Maryland Human – Computer Interaction Lab.  The QUIS is currently at Version 7.0 with demographic questionnaire, a measure of overall system satisfaction along 6 scales, and measures of 9 specific interface factors.  These 9 factors are: screen factors, terminology and system feedback, learning factors, system capabilities, technical manuals, on-line tutorials, multimedia, teleconferencing, and software installation.  Currently available in: German, Italian, Portuguese, and Spanish.

Object Oriented Input System: OIS (Object-Oriented Input System) is a code library for constructing a human-computer interface with input devices such as a keyboard, mouse or game controller.  OIS is designed so that software developers can easily use input from these devices with a computer application.

Aza Raskin: Aza Raskin (born February 1, 1984) is an American interface designer.  He is the son of human-computer interface expert Jef Raskin.

Engelbart's Law: Engelbart's Law is the observation that the intrinsic rate of human performance is exponential.  The law is named after Douglas Engelbart, whose work in augmenting human performance was explicitly based on the realization that although we make use of technology,

Archy: Archy is a software system whose user interface poses a radically different approach for interacting with computers with respect to traditional graphical user interfaces. Designed by human-computer interface expert Jef Raskin, it embodies his ideas and established results about human-centered design described in his book "The Humane Interface".  These ideas include content persistence, modelessness, a nucleus with commands instead of applications, navigation using incremental text search, and a zooming user interface (ZUI).  The system was being implemented at the Raskin Center for Humane Interfaces under Raskin's leadership.  Since his death in February 2005 the project was continued by his team, which later shifted focus to the Ubiquity extension for the Firefox browser.

Purple Numbers: Purple Numbers is a suite of tools that allows one to address paragraphs of HTML content.  It was originally conceived by Douglas Engelbart for his creation of hypertext.  The name comes from the faint, purple numbers at the end of each paragraph, which represent the link to that paragraph.  The name was coined by Christina Engelbart, Doug's daughter.

Supporting paragraphs: Purple Numbers Douglas Engelbart
Answer: mouse