We mainly focus on the official OpenSolaris distribution in this blog (with it's first release 2008.05 and heading to the second release 2008.11). But there are some other distributions of OpenSolaris which are built on top of the code from the opensolaris.org project. I've been asked more a couple of times about the differences, so let's take a closer look at these distributions and compare them.
Distributions created by Sun:
1. OpenSolaris - it's the "official" OpenSolaris distribution, developed & supported by Sun. If you are new to OpenSolaris, this is what we recommend to use. You can download the Live CD and install the distribution from the live CD. Unlike some other distributions OpenSolaris uses ZFS as the main file system and comes with IPS which is a network-based package system (similar to apt-get or yum). It provides access to the newest technologies and is targeted on desktop & laptop users, so that people can experience it on common hardware (not only on the server). It also has some customizations which make it easier to use for Linux users - e.g. gnu tools and bash are available and you can get lots of familiar software from the IPS repository. OpenSolaris 2008.05 can be installed only on x86, however future releases are planned to provide SPARC support as well.
2. Solaris 10 - this is the original Solaris distribution that existed before Solaris has been opensourced. It's the most stable distribution targetted to servers and is used in mission-critical deployments. It is a distribution whose goals are stability, scalability, realiability and high security. Continuous updates are provided to Solaris 10 to provide access to newer technologies. Solaris 10 is what you want to run your business applications on. It runs both on x86 and SPARC. This distribution has the longest support cycle.
3. SXCE - Solaris Express Community Edition - this distribution is a bi-weekly binary release created from the OpenSolaris source code. It provides access to the newest bits, and was the base for the OpenSolaris 2008.05 release. It supports both x86 and SPARC. It doesn't have some of modifications done in OpenSolaris 2008.05, such as IPS, LiveCD and Linux customization, so it looks more likes the original Solaris distribution, however you can try the bleeding edge improvements. This distribution is not supported. Some of the folks who have been using Solaris for a long time prefer this distribution over 2008.05 because it is more familiar to them. However for newcomers we rather recommend to try 2008.05, especially if you have any Linux background.
4. SXDE - Solaris Express Developer Edition - this distribution existed for some time as a special distribution created from SXCE with developer tools but it has been replaced by OpenSolaris 2008.05. OpenSolaris 2008.05 contains many developer tools including gcc, Sun Studio, debugging tools, AMP stack and more in the IPS repository. You can still download an older build of SXDE in case you want to give it a try, but future development will be done on the OpenSolaris distribution.
Distributions created outside of Sun - by the community:
1. SchilliX - A fully open source distribution based on the OpenSolaris source code. The first version of the distribution was created one week after Sun opensourced Solaris, so it was the first distribution created by the community. It is a LiveCD distribution.
2. BeleniX - BeleniX is an OpenSolaris Distribution with a Live CD (runs directly off the CD). It includes all the features of OpenSolaris and adds a whole variety of open source packages. It can be installed to harddisk as well and can be also booted off the USB memory stick.
3. MilaX - the goal of MilaX is to provide a "small" version of Solaris - you can use it even if you have less memory (256 MB for graphical login and 128 MB for command line). It supports both x86 and SPARC. It provides lots of popular open source software and can be installed on storage media with small capacities, like bootable business cards, USB flash drives, various memory cards, and Zip drives.
4. Nexenta - the special thing about Nexenta is that it uses Debian's APT tool for package management. It uses Solaris' kernel but provides many popular Linux packages. Version 1.0 of the NexentaOS was released in February of 2008. Nexenta is a distribution that combines Debian userland with the OpenSolaris kernel.
5. MartUX mBE - BlastwaRe Edition was the first non-Sun OpenSolaris distribution that was available both for SPARC and for x64/x86.
As you can see there are quite a few choices in case you want to give OpenSolaris a try. Again, if you are new to OpenSolaris I'd recommend to start with OpenSolaris 2008.05 but if you have any special requirements various different distributions with different features are available for you as well.