My biggest regret is a miter saw, but not just any miter saw, a particular one that is often advertised for $138.99 from a less-than-stellar tools supplier. It's a 12" dual bevel compound sliding miter saw, and it has great capacity to make cuts.... HOWEVER, accurate cuts are damn near impossible. When at rest, if you have everything true, it looks like your cut is going to be perfect. When you pull the saw down and it changes the bevel by almost a degree, it makes mitering a huge chore. Don't ever buy this POS.
S positive pressure ventilation mask. The mask works great but it will not work with glasses which I need to see close up. They will also not work with safety glasses, only a full shield.
Potter Cable dovetail gig. Never used to much to set up correctly. Starting to play with the idea of doing by hand. D. Barron dovetail guide and #372 Japanese saw, practicing.
Barrel handled saber saw, I always thought I wanted one but I don't like it at all!
Definitely the dovetail jig for the router. Used it once. Put it back in the box, hasn't moved since.
A biscuit joiner. It is a well known quality brand, but joints were too sloppy. Then I discovered the Dowelmax dowelling jig and use it all the time for edge gluing boards, joining cabinets, etc. Dowels are fantastic IF you have a quality jig.
I bought a "detail sander" 15 years ago, with adhesive backed, "triangular sandpaper pieces". The thing lacked power and really only vibrated somewhat. The paper peeled off on the edges easily. Junk.
Open end drum sander. Useless if you want a flat board with no taper.
A Stanley Honing Guide was so difficult to clamp a blade square in the guide. It didn't matter whether it was a plane blade or chisel
Plate jointer aka biscut joiner