As you're probably figuring out, I'm a long-time Linux fan. Unlike many other OS's, Linux doesn't have a single version. In fact, Linux itself is only a kernel. Other people take this kernel, bundle it with many other apps, and publish what is known as a distribution. As a Linux user since 1997-ish, I've tried many different distributions and have seen many come and go over time. Having dabbled in all things Linux for the better part of a decade, I've pretty much settled on Debian-based distributions whenever possible. (This has been the case since October, 2000.)
Unlike all of the other distributions, Debian is not a commercial entity. This makes for a very different release, with much different priorities. For instance, Debian tries to treat all Linux platforms equally, making it particularly well-suited for use on non-x86 architectures. Debian also prides itself on very stable releases. As it must co-ordinate releases on 11 architectures, the release cycle is much slower than for most Linux distributions.
Since most Debian users are long-time Linux users, areas such as installation tend to be a little less straightforward than say Mandrake, while development and system maintenance are far easier on Debian than any other OS I've used.
These features make Debian an excellent base distribution for commercial companies. Over the years, many Debian offshoots have come and gone. A few of the notable ones are Xandros, Lindows, Progeny and Storm Linux. As of 2007, the most successful Debian-based projects are Knoppix, Xandros, Linspire and undoubtedly Ubuntu.