Siril 0.9.0 rc1 is the first release-candidate of the new version (2012-2015) of Siril. It is mainly a corrective update since the previous version 0.9.0 beta, which was itself a massive upgrade of the previous 0.8 version of 2006 (old website). Changes in this rc1 version over the beta include:
many fixes including background extraction, compositing alignment, rejection algorithms, wavelets
threading of the heavy computations, to avoid graphical user interface freezing and provide a nice way of seeing what is happening in the console window
image rotation with any angle (not in registration yet)