I've used half of my 30 GB drive. Most of that is with OS X/OS 9, Applications, and MP3s/MPEGs.
I don't do any video editing, so I don't need space for that. I do web development and do some work with relational databases, but those take next to no space to set up (few megs at most). It's the servers where I deploy to that need the monster drives (single databases exceeding 4 GB -- hundreds of thousands of records).
Really the only thing that I need much space for is full game installations. But it's usually possible to delete an old/unused game to make room for the new one. I can't wait for my Sacrifice pre-order to arrive!