I like programming contests and have participated in multiple ones, like the Dutch Programming Championships (now known as BAPC) which we won in 2001 and 2003, the ACM regionals and Topcoder online contests. I was part of the organization of the 2004 Dutch Programming Championship and for this we also developed an open-source judging system: DOMjudge. In 2007 and 2008, we organized the Northwestern European Regional Contest (NWERC) for the ACM International Collegiate Programming Contest. Since 2012 I am involved in the ACM World Finals, where DOMjudge is used as judging system.
Besides that I also did system administration at my Study Association and for some computers in the students room at the theoretical physics group. Both sites ran Debian GNU/Linux, which is a great Linux distribution, especially to maintain.
I am a great fan of Free/Open Source software and wrote some myself. Here is a page links and downloads for some miscellaneous software I wrote that other people might find useful.