When i installed Gnome Desktop, how i can also easilly install various linux Apps like internet browser? If possible from within Gnome desktop, how to install visual package manager on CentOS 5.x (Redhat)?

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Edit:

Some softwares can be installed from command line.

Here is some list of package managers (software to manage software):
synaptic
aptitude
just install from command line like centos (yum install synaptic) or debian based (apt-get install synaptic)