The default interactive shell is tcsh. Recently I discovered that this shell also has the ability to complete your command-line by using the built-in complete command. After some experimentation, I’ve added the following lines to my /etc/csh.cshrc file:

complete cd 'p/1/d/'
complete man 'p/1/c/'
complete git 'p/1/(add am checkout commit diff format-patch gc init log mv rm revert status)/'
complete openssl 'p/1/(enc rand base64)/' 'p/2/(-base64 -aes-256-cbc -idea-cbc -bf-cbc)/'
complete portsnap 'p/1/(fetch)/' 'p/2/(update)/'
complete portmaster 'p/1/(-a)/' 'p/2/(-B)/' 'p/3/(-d)/'
complete shutdown 'p/1/(-p)/' 'p/2/(now)/'
complete df 'p/1/(-h)/'
complete du 'p/1/(-csm)/'

This means e.g. that if I type cd followed by the TAB key, the shell will list all available subdirectories. For another example, if I type portsnap and press the tab key twice, it expands to portsnap fetch update. Especially with programs like openssl and git which have a ton of available commands, this comes in very handy.