iPod 60 GB shows 27.6 GB free space available, but there's really only about 19 GB available, according to iTunes and Anapod Explorer. Given that the 27.6 GB number, as shown from the iPod has not changed in months, even when adding / deleting files, and the other values as shown by iTunes and Anapod Explorer (which are similar, but not identical values) do change I suspect the information shown from the iPod is inaccurate.
I updated the firmware on 2006-08-23 (from 1.0 to latest - 1.1.2) to see if it would correct the situation, but the anomaly remains.
iPod 60 GB shows: Settings -> About: Songs 2069 -- Capacity: 55.6 GB -- Available: 27.6 GB
iTunes shows: Items: 2069 , 50.7 days, 36.84 GB -- Used Space: 36.94 GB -- Free Space: 18.74 GB
Anapod Explorer shows: Count: 2069 (files) -- Used Space: 37,717 MB (37 GB) -- Free Space: 19,183 MB ( 19 GB)
Each shows the same number of files, but only iTunes and Anapod Explorer reflect similar values for free space, at approximately 19 GB, while the iPod shows approximately 27.6 GB free.