/l (lower case L) logs the user off
/s shuts down the computer
/r reboots the computer
/g reboots the computer and restarts any registered apps
/a aborts a scheduled shutdown or reboot
/p shutdown the machine with no timeout or warning
/h hibernate the machine
/e document the reason for the shutdown
/m specifies the machine to work with
/t set the timeout of the shutdown or restart
/c command on the reason for the restart or shutdown
/f force running apps to quit
/d provide a reason for the shutdown

Lots more information can be found about these switches by running shutdown /? from a command prompt.

(These switches are from the Windows Vista version of the shutdown command. Older versions have less/different switches.)