SUSE Linux Enterprise / openSuSE: openSuSE is the community version of SUSE Linux Enterprise. Survivors from the Nineties, the SUSE distros have suffered from the fallout of being bought by Novell, and then Novell's infamous agreement with Microsoft in November 2006. openSUSE, in particular, never seems to have developed the close community that its technical quality deserves. However, SUSE Linux Enterprise remains one of the most successful commercial distributions, and both distros include unique features such as the YAST configuration tool and the SLED menu for GNOME.
Ubuntu: In less than four years, Ubuntu has come out of nowhere to become the dominant desktop distribution. In the Linux Foundation's Client/Desktop Survey for 2007, its various incarnations accounted for 55% of desktops. Part of this success is due to its building upon Debian, but it is also probably the most innovative distribution today, especially when it comes to usability.
Most second tier distros are more unstably positioned than first tier ones. Some distributions in this category are strong contenders for the first tier, but, though they might eventually get there, are still finding their own level. Others are long-established, and have attracted stable, medium-sized communities without being the sources of innovation or spinoffs that their first tier counterparts have. In addition, the challengers are less likely than the giants to be equally successful in developing both a business and a community.
CentOS: Based on the Red Hat Enterprise Linux source code, CentOS could have become first-tier, except for the long delay of its last major release. CentOS's appeal lies mainly in its compiling of Red Hat source code into freely available binaries and its reputation for testing, since its versions go through three levels of testing -- Fedora's, RHEL's, and its own. In the four years of its existence, it has hovered consistently around number 15 in the Distrowatch list.
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Linspire / Freespire: Freespire is the community version of Linspire. Linspire has attracted considerable attention, but much of the attention has been of the wrong sort, including outrageous statements by CEOs like Michael Robertson and Kevin Carmody, and questioning of its technical decisions, such as the relaxation of the distinction between root and everyday accounts. Nor has its proprietary CNR repository proved a hit, though some hardware bundling deals seem to have established a small Linspire community. As for Freespire, it is too recent to have attracted much notice.
Linux Mint: Based on Ubuntu, Mint attracted considerable attention on its first release, mainly because of its emphasis on desktop usability. Each new release continues to attract much the same attention. But in between releases, its popularity slips considerably, suggesting that its a distribution that users love to try, but that comparatively few stay with.
MEPIS: A few years ago, when Debian had the reputation of being difficult to install, distributions like MEPIS emerged to provide the Debian experience with an easier install. This need no longer exists, but MEPIS still retains its partisans.
PCLinuxOS: Of all the second tier distributions, PCLinuxOS is currently the one most likely to become first tier. In the last six months, it has consistently topped the Distrowatch download list, thanks mainly to a user-friendly desktop that borrows from most of the first-tier distributions, but particularly Mandriva and Debian/Ubuntu. However, whether it can sustain this recent popularity remains to be seen.
Xandros: Xandros is the descendant of Corel Linux, which in 2000 was one of the first-tier distributions. But, despite an emphasis on servers in the last couple of years, Xandros has been unable to capture the imagination of most of the community.