Hmmm, is this HN worthy? These commands are so basic that I don't expect any HN-reader using Unix systems not to know those.
What about some slightly less basic ones that I'd think would be useful for many people.
# less with syntax highlighting
alias less="/usr/share/vim/vimcurrent/macros/less.sh"
tailf # tail -f, but better & shorter
mtr # check your connection (ie., traceroute with more info)
htop # nice a colorful process listing (better than top)
locate # search files by name -- that late-night disk thrashing is useful after all!