He developed Jolt-C, the ping of death. He worked for Microsoft on Windows 2000, testing for interoperability with Unix systems as his specialty.
First smart denial-of-service attack.
http://techcrunch.com/2008/07/22/top-10-best-ever-hackers/
http://www.zdnet.com/news/hacker-by-day-cracker-by-night/95887