Requirements: command line (any OS), must be extremely useful, very cool, and does not have to be a utility (eg, can be a CD ripper or CD burner whatever). Please also include an example favorite usage of the tool that illustrates it's usefulness.
I'm just going to include my favorite command line utility, but feel free to include your top (up to) three if you wish! Looking forward to finding some cool new tools from this thread!
lftp: What can't you do with this ftp client? It can do ftp, ftps, http, https, hftp, fish, sftp and file. It even supports torrent protocol. It supports a subset of standard unix commands as well.
Favorite usage: quickly mirror a remote directory to current local directory:
lftp -e "mirror -v /path/to/folder" sftp://user@host