OpenSolaris 2008.05 is the first official binary release. It was also called project Indiana.
The most important features are Dtrace monitoring, ZFS filesystem, VirtualBox for virtualization and the LiveCD function.
OpenSolaris uses the Gnome GUI which is the same GUI as Ubuntu and other Linux Distro`s.
The five reasons why i will choose OpenSolaris 2008.05 instead of Linux for my next webserver;
1. The price (free)
It`s free and its Open Source. Using the software on multiple servers doesn`t cost you a penny more!
It`s possible to order a free (bootable) DVD with OpenSolaris 2008.05 on it (click here to order the dvd)
Because OpenSolaris is based on Solaris (which is based on Unix System V, like IBM AIX and HP-UX) it`s reliable.
Solaris is the number 2 most reliable OS with uptimes over 99,98% (if you don`t believe me better check out these links; http://blog.linuxoss.com/2007/05/linux-versus-windows-os-impact-on-uptime-and-speed/ )
Sun contributes to a range of open source projects, including their productrange MySQLTM, NetBeansTM, GlassFishTM, OpenOffice.orgTM, OpenJDKTM, java.net, and many others which you might have heard of.
With the monitoring and analysis tool Dtrace (which stands for Dynamic Tracing)
DTrace provides the observability to accelerate application development and optimization of Apache/MySQL/PHP (AMP) and MySQL/Apache/Ruby/Solaris (MARS) stacks which run on the system.
4. Solaris ZFS Filesystem
ZFS stands for Zettabyte file system, which is Sun`s way of datastorage. Ever wanted a filesystem which doesn`t need maintenance or is 128 bit?
Unlike other filesystems it has been designed with the idea in mind that all disks will eventually fail. It has excellent data-loss detection mechanisms and can even detect silent data corruption (see the ZFS end-to-end data integrity post).
The thing that I like about ZFS the most is the freedom when assigning resources to different mount points. With other file systems I was always limited about how I structured my partition table (e.g. space for /usr, space for /home etc.). With ZFS there are so called pools and all the mount points share the same pool, so it is much harder to get out of space. (Quote from The Observatory, Sun Blog)
OpenSolaris is also Xen-hypervisor enabled as both a virtualization domain and guest.
As a separate free download, Sun also provides VirtualBox as host-based virtualization for Linux and Windows compatibility. I recommend you to try VirtualBox (version 1.6.2) if your going to experiment with virtualization since its totally free.
And if your going to switch to OpenSolaris I recommend checking out these sites;