Bookends - a bibliographic reference manager. Very actively developed; e.g., just come out with a killer feature: double-tap the ⇧-key in your word processor (Word or Scrivener, in my case) and you get a mini-search-bar to search for an article/book, select from the best matches, and paste the citation in-line in what you’re writing. Great for keeping the flow going. I keep all my PDFs in an iCloud folder that is linked to Bookends, so I don’t have to pay for storage (beyond what I pay for iCloud). Syncs fast, also with the iPad, where it has its own (pretty good) iOS app.
HoudahSpot - for turbo-charged searching, and especially for the creation/export of saved searches as “Smart Folders” (for more on this see my discussion with David & Katie on MPU #245
FoxTrot Professional Search - for industrial-strength search. It’s expensive, but it finds everything, even in packages.