Find and extract all rar files in subfolders into the current folder:

find . -name *.rar -exec unrar x {} \;

Find duplicate files:

find . ! -empty -type f -exec md5sum {} + | sort | uniq -w32 -dD

Find duplicate filenames:

find . -mindepth 1 -printf '%h %f\n' | sort -t ' ' -k 2,2 | uniq -f 1 --all-repeated=separate | tr ' ' '/'

Find files older than 7 years:

find . -mtime +2555 -print

Find directories containing files older than 30 days:

find . -type f -mtime +30 -printf '%h\n' | sort | uniq > old.txt

Find directories containing ONLY files older than 30 days (not working with subdirs properly):

find . -type f -mtime +30 -printf '%h\n' | sort | uniq > old.txt
find . -type f -mtime -30 -printf '%h\n' | sort | uniq > new.txt
grep -vf new.txt old.txt