The current development version is denoted as 1.9.
Compared to the stable version, the development version is more bleeding-edge and usually contains features that can not be found in the stable version - either because development has simply commenced after the stable release, though sometimes features deemed too experimental are removed from a release.
Development versions are "unstable" both in an API sense: when a feature is in development, it isn't uncommon that the interfaces to utilize it change rapidly in a short time. Also, while we try to have the Crystal Space development version working all the time, the work-in-progress nature makes it unavoidable that compilation is broken on some systems or instabilities occur at runtime.
This means that using the development version sometimes require frequent updating of Crystal Space itself to get relevant updates and fixes; it also means that the source code of your application may have to be changed frequently as interface changes occur.