The world of open source is big. Although Solaris releases do contain the software companion disc, there’s always more projects out there that can’t fit there.
OpenCSW fills this gap by providing binary packages, together with their build recipes. The packages can be installed comfortably with automatic dependency resolution, and require root access to the machine. If you want to compile and build packages on your own, the build recipes are available for all packages too.
Solaris 10 is fully supported. Solaris 8 and 9 only get best effort support level. As of January 2012, Solaris 8 catalogs do not get any updates, and Solaris 9 catalogs only get some amount of updates.