ssh -nNL33060:127.0.0.1:3306 username@host

or to background the tunnel so you can use the terminal window for other commands (make note of the process ID it returns so you can “kill” the tunnel when you are finished with it):

ssh -nNL33060:127.0.0.1:3306 username@host &> /dev/null &

Notice the use of local port 33060 instead of 3306; this works even if you have a local mysql server running on your machine, but means you have to connect to the remote server using local port 33060, not 3306. (On the remote end, connection is to port 3306.)

Note: If trying to connect to a mysql server on a machine running Mac OS X Server, the “allow network connections” checkbox must be checked in Server Admin for tunneling to work with GUI tools.