Ask HN: What's a Good Gift for Programmers?

What's a useful gift to give to a programmer? I am trying to think of something that they will either use daily, or increase their knowledge/productivity. Budget $100USD. I was thinking one-off/non-subscription high-quality paid utilities, e.g. diff tools such as Beyond Compare, HFS+ file system driver etc. I also considered courses/ebooks but generally I feel that, unless these are the highest quality, utilities will be more often used. Anyway, all suggestions welcome, regardless of targeted developer platform/domain.