Mac OS 7 to Windows 7: OS Showdowns Through the Ages!
Mandriva Linux - Circa 1998
Mandriva's now familiar goal was to make "a Linux distribution that would be as easy to use as Windows." The very first release borrowed heavily from Red Hat Linux, which shared a similar goal, and it shipped with KDE 1.0 as the default graphical environment.

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Today Mandriva remains a viable option for users new to Linux while also boasing large hardware support by default. However, it's also one of the 'heavier' Linux distros available, making it somewhat sluggish on older PCs.
Knoppix - Circa 2000
Every IT worth his salt should be familiar with Knoppix, the Debian-based OS designed to run from a Live CD or Live DVD. While Knoppix doesn't present a threat to Windows (it can actually be installed on a hard disk), it does find itself in every serious IT's toolbox for troubleshooting on the go.

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Knoppix was first released in 2000 and is considered by many as the original Debian-based Live CD. There have been other versions of Linux early on that were able to run as demos and rescue disks, but superior hardware detection and automed configuration scripts gave Knoppix a leg up on the competition.
Linspire - Circa 2001
You may better remember Linspire as LindowsOS, the controversially-named OS based on Debian GNU/Linux (and now Ubuntu). As the name implies, Linspire was developed as a commercial alternative to Windows capable of running Windows programs. It initially tried to this with with Wine before later shifting its focus away from Windows and towards Linux programs.

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Microsoft filed suit against Lindows in 2002 for obvious reasons and initially lost the court fight. Before a retrial could take place, the two sides settled out of court with Microsoft paying roughly $20 million to buy the Lindows trademark. Since then, it has been called Linspire.
Fedora - Circa 2003
Another RPM-based distro like Red Hat and Mandrake, Fedora picked up in 2003 where Red Hat Linux left off (Red Hat Linux discontinued its focus on the desktop and proceeded forward with Red Hat Enterprise Linux).

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Fedora has always been largely a community-driven collaboration containing only free and open-source software.The first release of Fedora -- Fedora Core 1 -- was based on Red Hat Linux 9 and shipped with the Linux 2.4.19 kernel and GNOME 2.4.0. It wasn't until Fedora Core 4 was released in 2005 that the Linux distro would support the PowerPC architecture.
And Then There Was Ubuntu
Ubuntu - Circa 2004
Sometimes it takes a perfect alignment of the moon and stars to get things just right. Take Linux, for example. In the battle for desktop supremacy, no one outside of the open-source faithful truly believed Linux had much of a fighting chance at snagging significant market share away from Microsoft, and some still don't. But Linux is closer than ever to becoming a mainstream no-cost alternative to Windows, a result of Vista's bug-prone initial release and Canonical developing an open-source OS so easy even Aunt Mabel could get through an install.

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This wasn't necessarily the case when Ubuntu was first released in 2004. On the Windows side, users had already become entrenched with XP, and Ubuntu lacked the overall polish and hardware support to convert an already content userbase. But once Vista shipped and the complaints started pouring in, users who previously never considered Linux began looking at Ubuntu, which was steadily improving and attracting more developer support (even Creative has jumped on board).
Ubuntu comes preinstalled with a variety of software, including OpenOffice, Pidgin, Firefox, GIMP, and a bunch more. If there's a Windows-specific app that a user simply can't live without, there's a good chance Ubuntu can still oblige via Wine.