I created a new user account that I needed for a specific program, but it doesn't represent a regular user. How do I prevent this user from showing up as an option on the login page?
Both in 10.7 and 10.6 you will need their shortname then just run this command in Terminal.app
$ sudo defaults write /Library/Preferences/com.apple.loginwindow HiddenUsersList -array-add user1 user2 user3
To remove them from the hidden user list:
$ sudo defaults delete /Library/Preferences/com.apple.loginwindow HiddenUsersList