Here is the best summary from http://coolestguidesontheplanet.com/clear-the-local-dns-cache-in-osx/:
For Yosemite the command is
sudo discoveryutil udnsflushcaches
For Mavericks, the commands are
dscacheutil -flushcache; sudo killall -HUP mDNSResponder
For Lion & Mountain Lion the commands are
sudo killall -HUP mDNSResponder
For Leopard & Snow Leopard the commands are
sudo dscacheutil -flushcache
It looks like the dscacheutil is going to be a handy utility for web developing. Woohoo!
Update: Another useful thing is how to restart remote desktop from a command line:
sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart -restart -agent -console -menu
Remember that, too.
Update II (2008-06-14 12:42 p.m.): Another useful thing is how to restart MySQL (or other startup programs) from a command line:
sudo /Library/StartupItems/MySQLCOM/MySQLCOM stop
sudo /Library/StartupItems/MySQLCOM/MySQLCOM start
Update III (2011-09-14 19:27:57): Another useful thing is now to compute directory usage from the command line. The syntax is a little different for OS X than other *nix-based languages. To display the usage of all subdirectories below the current directory (“-d 1” means depth of one):
du -h -d 1 ./
I upgraded my Mac to Leopard yesterday and it is a beautiful operating system. My silver G5 runs fairly well, but bootup is considerably slower now. I was making some network changes today and had the need to flush my Mac’s DNS cache but had a hard time finding out how (without having to reboot, that is.)
After some frustrating searches (no, there is no lookupd command in Leopard) I finally found the answer thanks to ITToolbox.