Mark Spencer (born April 8, 1977) is an American computer engineer and is the original author of the GTK+-based instant messaging client Gaim (which has since been renamed to Pidgin), the L2TP daemon l2tpd and the Cheops Network User Interface.

He is the founder, chairman and CTO of Digium, an open-source telecommunications supplier most notable for its development and sponsorship of Asterisk.

Spencer did not have enough money to buy a PBX (private branch exchange) for his company so he decided to write Asterisk and later founded Digium.

Adapting a similar strategy as Asterisk, he developed the eXtensible Flight System, XFS, a cross-platform avionics architecture.

XFS has already been integrated in the Zenith CH750 STOL aircraft in the form of both a three-screen panel and the "Unpanel," a portrait-orientation (also landscape) screen to replace the entire traditional glass cockpit.