If you're a bit like me, a deep down UNIX guy, and an OS X guy you've probably monkeyed with ipfw. I have several times played around with the firewall rules, testing different things, experimenting and whatnot. Unfortunately in doing so, the Firewall pane of the System Preferences refuses to work anymore saying that there is some other firewall software running. Flushing ipfw doesn't help matters either. Several times in the past I resorted to rebooting to restore my ipfw rules. And since I am a deep down UNIX guy I hate rebooting even more than not using UNIX. Well, no need to reboot anymore. I found
/usr/libexec/FirewallTool which dutifully restores the OS X specified rules so that the System Preferences will stop complaining.