JFact is a Java port of the FaCT++ OWL DL reasoner; its design matches FaCT++ design very closely, except for the low level details where Java offers different ways of doing things, so if you are knowledgeable of FaCT++ internals you'll find that many of the JFact classes and methods look familiar.
FaCT++ and JFact also share the same license; JFact is therefore available under the Lesser GPL license.
Note that JFact does not include some parts of FaCT++: the DIG interface and the command line support are absent.
User feedback about bugs and desired features is very welcome, as well as architectural criticism; the code base is fairly complex and it is very likely that not all design decisions that have been taken are the best ones; so, if you feel like digging in the code and suggesting fixes or optimizations, please do so.
A simple example on how to use JFact through its OWLAPI interface is available here.
JFact performances are currently under evaluation using the OWLReasoner Benchmark Framework.