OS X Command Line Tools

Obviously, i mean OS X-specific tools, not the unix stuff.

• open - "open files and directories", like you double-clicked on them, or dragged and dropped onto a specific app
• ditto - "copy files and directories to a destination directory", ie Mac-savvy cp, which knows about resource forks and HFS metadata and such; will also read and write zip and cpio archives, and has a few bells and whistles (although many handy things cp can do are missing)
• hdiutil - "manipulate disk images", see also hdid
• lsof - "list open files"; this one's from unix, i think, but it's a hell of a tool nonetheless, hell of a tool (key flags: -p selects a particular process to list, -i lists only network sockets, and most importantly, -a causes the selections to be ANDed rather than ORed, which would be stupid)
• ps - not even remotely an OS X-specific tool, but something you should know is that you need to say ps -x to list all your processes, rather than just ps, because by default, ps doesn't list processes that aren't attatched to a terminal, which on a GUI OS like OS X is most of them
• osascript - "execute AppleScripts and other OSA language scripts", ie applescripts only
• say - "Convert text to audible speech", hours of fun!
• enable, disable - "stop/start printers and classes", could be handy
• GetFileInfo - "get attributes of HFS+ files"
• lsbom - read Bill Of Materials files, as used in the official OS X installer
• bless, softwareupdate, screencapture

Also, the bash version of where is whereis.