WSJT-X_improved is a derivative of the original WSJT-X which is programmed by DG2YCB. Its software offers some improvements in operation and some extras concerning the decoder. An overview can be found at: https://sourceforge.net/projects/wsjt-x-improved/
There are different versions, the download and the notes to the revisions can be found at: https://sourceforge.net/projects/wsjt-x-improved/files/
DG2YCB is now also a member of the WSJT-X development team. That lets expect that there will be meaningful additions in the operability of the software.