Seems like computer stuff is popular--I've bought quite a few things on Newegg, including a new PC, headphones, and a few other things.  And while not endorsing this, you can buy gray-market pharmaceuticals like nootropics and kratom with bitcoin, though I don't know how popular that market is.

I think what people really want is for big retailers to start accepting bitcoin, e.g., Amazon, eBay, and other places where they tend to shop the most.  Overstock.com does take bitcoin, but I haven't found much on that site worth buying.  They don't even sell cases of cat food last time I checked, and most of the writing instruments I was looking at were kinda crappy.

Never used Tor or bought anything on the dark web, but I'm pretty sure bitcoin is very useful there.  Not surprising, but it'd be nice if more big online merchants started taking it.  I'm not a big proponent of using bitcoin as a currency, but there have definitely been times when I've had more bitcoin than cash and I've chosen to spend bitcoin on things. 

Cat food.  I'd really like to be able to buy wet cat food in bulk.  If there's a place that takes bitcoin where I can buy some, I'm all ears.