If you want a single word that can make up a section heading, then Clients or perhaps even better External clients is probably a good fit. "Client" is a reasonably well-understood term to mean "someone who is the final receiver of the product created", but unlike Lascivious Grace's suggestion of freelancing need not imply any particular form of employment.
However, I'm wondering if it might not perhaps be even better to put this into your regular employment history section, rather than as a section of its own? In which case, you could write something like (note that this uses the same word, just not as a section heading):
2009 -- 2013, Acme Designs, Corp., graphics designer and web developer, with clients including Coyote Hospital, Inc. (2010 -- 2013); Funny Movies, Corp. (2009); and Acme Explosives, LLC. (2009 -- 2011)
You could then break that up (bullet point list, newlines, ...) however you feel fits your resume's style best.