WordNet (r) 3.0 (2006):

for free
    adv 1: without payment; "I'll give you this gratis" [syn:
           gratis, for free, free of charge]

The Jargon File (version 4.4.7, 29 Dec 2003):

for free
 adj.

    [common] Said of a capability of a programming language or hardware that is
    available by its design without needing cleverness to implement: ?In APL,
    we get the matrix operations for free.? ?And owing to the way revisions are
    stored in this system, you get revision trees for free.? The term usually
    refers to a serendipitous feature of doing things a certain way (compare 
    big win), but it may refer to an intentional but secondary feature.


The Free On-line Dictionary of Computing (30 December 2018):

for free

   Said of a capability of a programming language or hardware
   equipment that is available by its design without needing
   cleverness to implement: "In APL, we get the matrix operations
   for free."  "And owing to the way revisions are stored in this
   system, you get revision trees for free."  The term usually
   refers to a serendipitous feature of doing things a certain
   way (compare big win), but it may refer to an intentional
   but secondary feature.

   [Jargon File]

   (1994-12-14)