Smart Folders in the Finder can be extremely useful. (I’ll leave for an other time my discussion of why I think Apple’s unnecessarily limited a feature that casual users will never use) Here’s a few really useful ones I use that you might like.
I love the idea of smart folders, but in practice I rarely use them. Every once in a while I create a one-off one to do a complicated search. Most files that I want to access together are already grouped in a BBEdit or Xcode project or in an EagleFiler library.