For lack of more to add at the moment, this is slurped directly and shamelessly from The Jargon File:

Share and enjoy! imp.  1. Commonly found at the end of software release announcements and README files, this phrase indicates allegiance to the hacker ethic of free information sharing (see hacker ethic, sense 1). 2. The motto of the complaints division of Sirius Cybernetics Corporation (the ultimate gaggle of incompetent suits) in Douglas Adams's "Hitch Hiker's Guide to the Galaxy". The irony of using this as a cultural recognition signal appeals to hackers.

-- LesOrchard - 29 Mar 2002