I need to run the web browser with another user but I don't want the user to be shown at the login screen. How can I create a user that will not be listed on the login screen? GNOME/Scientific Linux 6.3. 
How to create a user that doesn't show up on the login screen?
login;login manager
Edit /etc/gdm/custom.conf and add or change the Exclude directive in the [greeter] section:[greeter]Exclude=nobody,alice,bobUsers alice and bob won't be shown on the list at the login screen but can still log in by typing their name and password (if they have a password).See more details in How to hide users from the GDM login screen?(it's mostly distribution-independent  some details might change, for example files may located in different places, and the threshold for system users is 500 on most Red Hat derivatives but 1000 on most Debian derivatives).