When Sun released Solaris under an open-source license and began distributing the binary edition free of cost, millions of users downloaded it. Some estimates claims as many as 10 million downloads so far. As nice as Solaris is, though, users accustomed to Linux or FreeBSD found it lacking in several key areas: system management and other useful utilities, a friendly interface and dependency-resolving installers. The cries of "oh, if only Solaris had the GNU toolchain, a friendly installer, all those thousands of great Linux applications, Linux's great hardware support, and Debian's aptitude, it would be perfect."
These cries did not go unheard, and thus the Nexenta project was born, thought with a slightly different perspective: "if only GNU/Linux had a better kernel." Currently, it runs only on x86 hardware, so SPARC users can't play with it. But you still get the major Solaris goodies DTrace, ZFS, and Zones are all supported. And you get the old familiar GNU/Linux goodies, such as the Bash shell, vi, gcc, make. Plus, your favorite Linux desktop environment or window manager should be able to install with it. Nexenta uses Ubuntu Linux, rather than "real" Debian, though it hopes to someday being officially included in Debian.
Nexenta is still in the early Alpha stage of development, so don't run it on production machines unless you enjoy unpredictable behavior and hauling out of bed in the middle of the night to fix things. The installer needs a lot of polish, and hardware support for things like networking devices, especially wireless, and peripherals like printers and cameras is lacking. There is also an ongoing controversy over the legalities of combining code licensed under Sun's CDDL with the GPL.
At this stage consider it experimental. But it doesn't hurt to install it on a test box and get acquainted with it, or even contribute to the project.