We live in central Maryland, USA, and pulled this off a tomato plant in our garden this morning (mid-summer). Any idea what these are? Not sure if they’re beneficial or parasitic, but I’d say the latter if I had to guess.
The shape of those looks more like caterpillar droppings (frass) than galls to me, though I've never seen them so green or shiny. Here's an image of tomato hornworm droppings, it looks quite similar to your picture: If the balls are firmly attached to the leaf then they are galls, if they can be dislodged, then poop is more likely.
Galls, the plants reaction to certain insects. Generally no action is warranted because of minimal harm to the host plant.