Find files that have been forgotten. This information comes from commands like 23.19.and . (If you use the MH email system, you can find mail messages that haven't been read or scanned in a long time.) You can save disk space by cleaning up unused files; see article
A system like this could automatically archive files to tape and delete them. It could have a personal "skip" list of files and directories to skip. And so on...
Check a directory to see which files are being read by programs, compilers, etc. This "sanity check" can help you debug programs by confirming which files are being accessed.
NOTE: Some UNIX systems, including versions of BSD and SunOS, do not update the access time of executable files (programs) when they're executed. To test yours, use ls -lu on a pure-executable file (not a shell script) before and after you run it.