Mac OS X 10.2, 10.3: Starting up in Safe Mode
Wait for tone, then immediately press and hold shift key after the startup tone, releasing on seeing the Apple screen appear. Forces a "directory check" (runs a version of fsck that's compatible with journaled file systems.
It takes quite a while to startup in safe mode. In theory one has to hold the shift key after the startup tone, I did it beforehand and it worked. You know you're in safe mode because you'll see a notice of that during startup.
After restarting shut down completely and power up again. I do this periodically for preventive maintenance.