Command line utility for identifying HFS+ compressed files and getting the sizes of them. Its output looks something like this:
$ afsctool -v /usr/local/bin/afsctool /usr/local/bin/afsctool: File is HFS+ compressed. File size (uncompressed data fork; reported size by Mac OS 10.6+ Finder): 85372 bytes / 85 KB (kilobytes) / 83 KiB (kibibytes) File size (compressed data fork - decmpfs xattr; reported size by Mac OS 10.0-10.5 Finder): 24417 bytes / 25 KB (kilobytes) / 24 KiB (kibibytes) File size (compressed data fork): 24433 bytes / 25 KB (kilobytes) / 24 KiB (kibibytes) Compression savings: 71.4% Number of extended attributes: 0 Total size of extended attribute data: 0 bytes Appoximate overhead of extended attributes: 536 bytes Appoximate total file size (compressed data fork + EA + EA overhead + file overhead): 25376 bytes / 25 KB (kilobytes) / 25 KiB (kibibytes)
I've updated afsctool and added in place HFS+ compression for files and folders, just be warned that you should make a backup before attempting to use it as I haven't had a chance to extensively test it yet.
The in place compression seems not to have any significant issues, but if you are compressing anything important then always include the -k flag just to be safe.