Very low on disk space on C drive

I just got a warning dialog that the C drive was dangerously low on free space. I checked properties and it showed the disk was nearly full. I can't imagine what would be taking so much space. I went through the programs in the Control Panel and uninstalled a bunch of programs that I rarely used, but it made only a small difference. I also ran the disk cleanup utility, but that didn't make much difference at all.

Is there some utility I can run to identify what is taking so much space?

I'm running Windows 7-64 with a 500 GB hard drive.

I back up all my data on the B drive, also 500 GB.

C drive:


B drive: