A project server that I've been meaning to do for some time is a small backup server, able to be located seperately from my main rack.
I came across the M710q at local auctions and eventually picked one up for under AUD$100.
Out of the box it is well specced, with one m.2 slot and one 2.5" drive... but I really wanted 2x 2.5" so I could run a raid1 pool for backups.
Enter a slew of additional purchases, modifying a sata splitter, and a couple hours on the 3D printer to prototype a solution: