It depends on what they are lacking. I bought my brother a very nice weed trimmer.
In general, I just give cash, but if you're looking for something unusual, sometimes the hints are right there on the registry. For example, we registered for some camping gear and people got us some awesome stuff that wasn't on the registry that was probably more useful than the stuff we listed, because they have been campers for years and they had some tried and true favorites.
I've also seen people give the couple their wedding invitation in a nice frame, which I poo-pooed originally, but now that I've been married 15 years, I really wish I had my wedding invitation framed in my home. Couples spend so much time over the font, spacing, wording and paper, and it's nice to look back on.