He was a NetBSD developer:
http://ftp.netbsd.org/pub/NetBSD/misc/repositories/git/authormap
http://mail-index.netbsd.org/source-changes/1999/04/07/0051.html
He was also a PostgresSQL developer:
http://www.postgresql.org/community/contributors/
He wrote a port scanner…
http://hpux.connect.org.uk/hppd/hpux/Networking/Admin/strobe-1.06/
… and other free software. In short, he is a programmer by training
http://en.wikipedia.org/wiki/Julian_Assange#Computer_programming_and_university_studies
