LEGO Digital Designer Alternatives
There are several editors which are based on the open LDraw part database.

I believe that the most popular are MLCAD and LeoCAD. I personally use LDD, so I can't speak to the quality of the instruction generation from these tools, but they do at least offer the ability to create instructions.

LeoCAD is probably your best bet for doing animations, and it is available for Windows, Mac, and Linux.

BrickLink has recently released Stud.io in open beta, and it appears to combine the best of LDD and LDraw - it works with the LDraw part database but will import LDD models and offers a similar interface to LDD as well as collaboration and integration with the BrickLink shops for purchasing the elements for your models.