These are the items that I found essential as a new beekeeper.
1 - Hive Beetle trap by dw2364. I use these throughout my hive and have NEVER had trouble with hive beetles. I have to print new traps each year, as the old traps get covered in propolis and become increasingly hard to clean.
2 - Ant Moats - there are several to choose, but I have been using file from jetpad. They print easily and are very sturdy.
3 - Entrance Feeder - Again, a few to choose from. The one I like the best is from The Headless Sourceman. Very well designed.
4 - Frame Hooks - Again, a few to choose from. I like the hooks from ckcampbell248.
Special mention - something I haven't tried, but worth looking into BEFORE you set up your hive:
5 - For a precision frame spacing, the frame spacers from fbonte_ look great. It looks like they will also help protect the rabbet from damage. I haven't put any in yet, so I don't know how well they can be cleaned around.
I have created a collection called "Beekeeping". It contains all of the objects from Thingiverse that may be of interest to hobbyist beekeepers. I have not included all bee-related files, just those related to hive care, wax working, honey wands, etc. If you see something I'm missing, please let me know.