My favorite new one that I have been using a lot:

  function cheat() {
      curl cht.sh/$1
  }

It queries the cht.sh cheatsheet of various Unix commands. 'cheat tar' prints:

  # To extract an uncompressed archive:
  tar -xvf /path/to/foo.tar
  
  # To create an uncompressed archive:
  tar -cvf /path/to/foo.tar /path/to/foo/
  
  # To extract a .gz archive:
  tar -xzvf /path/to/foo.tgz
  
  # To create a .gz archive:
  tar -czvf /path/to/foo.tgz /path/to/foo/
  
  # To list the content of an .gz archive:
  tar -ztvf /path/to/foo.tgz
  
  # To extract a .bz2 archive:
  tar -xjvf /path/to/foo.tgz
  
  # To create a .bz2 archive:
  tar -cjvf /path/to/foo.tgz /path/to/foo/
  
  # To extract a .tar in specified Directory:
  tar -xvf /path/to/foo.tar -C /path/to/destination/
  
  # To list the content of an .bz2 archive:
  tar -jtvf /path/to/foo.tgz
  
  # To create a .gz archive and exclude all jpg,gif,... from the tgz
  tar czvf /path/to/foo.tgz --exclude=\*.{jpg,gif,png,wmv,flv,tar.gz,zip} /path/to/foo/
  
  # To use parallel (multi-threaded) implementation of compression algorithms:
  tar -z ... -> tar -Ipigz ...
  tar -j ... -> tar -Ipbzip2 ...
  tar -J ... -> tar -Ipixz ...

You do have to be online, but there are ways of downloading the directory of cheat sheets to use offline as well.