Sophos is no longer maintaining Sandboxie and has released the source code so others can pick it up and develop it further. Currently there are two main forks of Sandboxie development now that the code is open source and has been released.
Sandboxie Open Source
Developed by Tom Brown (former Sandboxie developer)
Tom's project leans more toward continuity, to keeping the software being what it is. No big changes. Conservative.
Development thread: https://www.wilderssecurity.com/threads/sandboxie-technologies-sbie-open-source.42815
Sandboxie Plus
Developed by David Xanatos (developer of PrivateWin10)
David's project is more liberal, he want's to make a lot of changes to Sandboxie.
Development thread: https://www.wilderssecurity.com/threads/sandboxie-plus-sbie-fork.427755
There are other major differences, Tom will sign his builds, David will not. His builds will get flagged as malware, and Tom's won't. With Tom's build, we will run our computers and use SBIE as we always have, with David's builds you might have to run your computers in test mode or boot with driver signing disabled.