Nightly builds are versions for debugging and testing. Those are built automatically every day with the latest code from the development repository.
They may contain changes which would make your savegames unreadable by any official version, or bugs that would destroy your game, or they may not even run at all. Use them at your own risk.
In case you want to use it anyway, remember to report any bugs you find.
Makeobj is a command line tool to create new objects for Simutrans. It joins image and data files into a single optimised pak-file that Simutrans can understand.