Here's an interesting one. I've written a few free-software apps in the past, and recently SpamAssassin has taken off. It's very much sysadmin-oriented, being a mail filter for spam which works well as a system-wide filter.
It's illustrated that there's a big difference in audiences, between app users and sysadmins; sysadmins will regularly hack the code to ''scratch their itch'' and send back a patch; whereas patches don't often come from users.
Interesting...