Disk Inventory X is fine, I also like another free one called
Grand Perspective. Unlike some others, I don't think endlessly upgrading to the next largest hard drive size is a viable long-term solution. The real solution is a modicum of file management.
For example: I have two iTunes libraries. A large one that's stored on an external drive, and a subset of my favourite tunes that's actually on the boot drive of my laptop. When I'm on the go, I don't need every song I own with me at all times, so I use the subset. When I'm home, I option-click and choose the larger library and have everything available to me. Saves a TON of HD space on my 320GB laptop boot drive.
You can do the same thing for photo libraries (if you use iPhoto), and movies. There's no need to carry everything with you all the time.