I'm going to be slightly flippant and say the the bundled tools and little one liners built with them. A few examples from my .profile follow:

Eject a volume:

  alias eject='hdiutil eject'

Copy the working dir to the clipboard:

  alias cpwd='pwd|xargs echo -n|pbcopy'

Show current airport status:

  apinfo='/System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport -I'

Use spotlight to search for a file:

  spotlightfile() {
      mdfind "kMDItemDisplayName == '$@'wc";
  }

Use spotlight to search file contents:

  spotlightcontent() {
      mdfind -interpret "$@";
  }

Display a man page in Preview:

  pdfman () {
     man -t $1 | open -a /Applications/Preview.app -f
  }

I've a few others but they're less generic than the above. Also although the above were sitting in my .profile I probably yanked them from random parts of the inter-webs; in other words, I take no credit for them.

EDIT: One more generic one I use fairly regularly:

  google() {
      python -c "import sys, webbrowser, urllib;   webbrowser.open('http://www.google.com/search?' + urllib.urlencode({'q': ' '.join(sys.argv[1:]) }))" $@
  }