I became very cavalier about my disk usage since I had so much and just 6 weeks into my Mac experience I was burning up over 240GB of disk space! Okay, a lot of that was video but there's a lot of other things in there too.
Paul, a commenter on one of my blog posts recommended that I try out Disk Inventory X, a tool for determining where all the space went. Disk Inventory X builds a graphical tree of where your disk space is being used and gives you some basic management capabilities while in there - which is deleting files for the most part.
It takes a little while to load - at least against the 240GB of files I had - because it rips through your entire hard drive looking for every file. Took about 2 minutes to display the main window, though it's plenty quick after that. There's a MacBreak Minute video the author hosts that walks you through the application - it's a link on his home page.
If you are low of disk space or are just wondering where all that free space went, check it out.