Startup Disk almost full - No Chance!!!

I've had my new iMac for about 2 months. I went with the 500 GB hard drive because I'm not doing anything crazy on mine, and got by with a G4 iMac for alomst 10 years with a fraction of the space and never had an issue.

I keep receiving an error message stating that my Startup disk is almost full.. There is absolutely no way this is right. I have a little under 11 GB of music, and roughly the same in pictures. That is pretty much the extent of what I have put on here.

I read some posts in the forums, and ran OnyX automation per someone's recommendation which seemed to free up a tiny amount of space but nothing that will help my situation. You'll notice below that my Mac shows only 60 MEGABYTES being available, and that 460 GB are being occupied by files in the "Other" category..

If anyone has any insight I would greatly appreciate it. Thanks in advance for your help.