My PowerBook G4's disk space seems to be rapidly disappearing. What do you think is wrong?

Time ("a couple of days") isn't the question. You could copy a 30GB folder from an external HDD in an hour and use up most of your available space.

These are the most effective steps (Do them in this order.) to recover HDD space:
-- Empty the trash. Many people forget about large files in the trash.
-- Open Disk Utility and verify the HDD. If problems are found, boot to the OS X install DVD and repair. A faulty file system will definitely cause space to be unavailable.
-- Copy all large files or medium size files that are in very large numbers (movies, photos, songs), to an external HDD, and then erase those same files from the internal HDD.
-- Use OnyX to delete log files. After a long period of use, the OS will develop log files can be several GB.
-- In Firefox preferences > Advanced, change the cache setting to 100MB. You don't need 10GB of cache. How often do you go back to Youtube and watch the same videos you watched two weeks ago? If you have a fast connection, you don't even need more than 50MB of cache.

you need to attempt and use spotlight to attempt and discover distinctive record types which you recognize you have used interior the previous for audio ie aiff etc and notice in case you will stumble on any that way and delete them in case you do no longer choose them, wanting that it will in all probability be much less complicated to wipe the force, re setting up mac os and re setting up the applications, make certain you backup your bookmarks so as that in case you have any sites you choose then you extremely understand think approximately a replica of them which you will re import into safari or firefox or despite browser you employ.

10GB of cache? You're going to a lot of sites with a lot of content. Probably also downloading a lot of things too.