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:]) }))" $@
}