The most popular and best Linux firewall is Iptables. IPCop, Shorewall, Uncomplicate Firewall, Checkpoint and OpenBSD PF are also on the most popular list.
Linxu is just the kernel, but I know what you mean, the most popular GNU/Linux distribution is Ubuntu
OpenOffice
Ubuntu is one "flavor" of Linux and one of the most popular variants for beginners.
One of the most popular Linux games of Trine 2. Another one that is popular is the role playing massively multiplayer game Heroes of Newerth. Another popular one is Xonotic.
There are many Linux distros. The most popular include; Debian which is 19 years old, Fedora, Ubuntu, Mageia and Mint Linux which uses the Cinnamon desktop interface.
First of all, it's GNU/Linux not just Linux(which is the name of the kernel). Second, most versions of GNU/Linux are free. Lastly, the most popular version is Ubuntu.
IPCop, a firewall software geared toward Linux SOHO users, can be downloaded from the IPCop website. It was created by Linux users who were unhappy with the existing firewall availability.
Because Linux is actually more popular in most fields these days than certified Unix.
There's Ubuntu, Debian, Red Had, Fedora, Gentoo, Arch, Mint, SuSE, Slackware... A good way to find out is to look into Distro Watch. The web site more or less keeps a monitor on how popular a given Linux distribution is.
There are several desktop environments for Linux currently in use. The most popular are GNOME and KDE.
Linux and Sun Micro
The most popular one is the Android OS.