Complementors are businesses that directly sell a product (or products) or service (or services) that complement the product or service of another company by adding value to mutual customers; for example, Intel and Microsoft (Pentium processors and Windows), or Microsoft and McAfee (Microsoft Windows & McAfee anti-virus).
Complementors are sometimes called "The Sixth Force" (from Porter's Five Forces model), a term which was coined by Adam Brandenburger.
See also Porter's five forces analysis
References Brandenburger and Nalebuff, Architecture Wins, 1998 Brandenburger and Nalebuff, The Value Chain, 1998 Nalebuff and Brandenburger, Co-opetition, 1995