The geotoolkit.org web site has very little documentation. From this document I get the impression that it is a fork from geotools; is that right? If so, what were the motives/objectives of the fork?
It looks like there was a fork that was intended to be reintegrated, but this never happened.
In response to the deteriorating quality of some GeoTools code base, Martin Desruisseaux embarked on a major cleanup effort. In July 2008, Martin created a new, initially empty, source code repository and proceeded to copy the GeoTools classes to this new repository one-by-one, cleaning and harmonizing them in the process. The project was codenamed "Geotidy" and intended to be offered as a new direction for the GeoTools project, probably starting the 3.x series of GeoTools.
During this discussion, it immediately became clear that the PMC was going to require a formal proposal and demand a vote over each change in this cleanup effort. The proposal to experiment various clones of the code repository (a Distributed Versioning System paradigm) did not get momentum. Consequently Geotidy developers decided to go their own way.
There is also an interesting mailing list discussion on GeoTookits application to become an OSGeo project (as well as GeoTools), fragmented communities, and various opinions on the divide.
We can't say which is the fork. martin desruisseaux made the referencing, metadata and coverage modules since 1996. geotools suffer a huge contribution dropdown in 2008 when the five developers leaved to make geotoolkit.
Both projects are equaly active. See:
Geotools just has the name. it's not even following the GeoAPI ogc spec anymore.
unlike geotools (which is nearly only working for geoserver), geotoolkit developers are working on several different type of gis project. - Constellation-SIG : map server - Mapface : JSF map library - MDWeb : metadata catalog - Puzzle-GIS : desktop application on netbeans platform