Good Morning, May I add the Roxanne thimble to the list here. I was never able to us a standard thimble though Mother tried to teach me to use one. After using the Roxanne thimble for about 5 minutes I was hooked. Can't sew without it now and I better understand what Mom was trying to tell me.
My second offering to the list is a sewing table that allows your sewing machine to drop down to table top level. That alone relieved so much pressure on my body that I can sew for much longer than I ever could with the machine on the table top.
Lastly, the biggest tool of all, my Pfaff sewing machine. Wow! What a difference their dual feed system made in my piecing. Can't piece without it now.
Oh, and the Martelli rotary cutters. They are much easier for me to use than the standard cutters.
Thanks to all the other suggestions mentioned here!