I recently inherited a much nicer table saw than the second hand Harbor Freight one I had before, and it was quite a revelation how much easier it was to get good results with it and how much safer it feels to use. I'd definitely agree on that one.

The other category where I like to spend more is tools I hate to use. Some things I have specifically for jobs that suck and anything that makes those jobs easier is worth it in my mind. I have a nice drain snake, and the most expensive toilet plunger I could find for this reason.