This inexpensive but well-designed app labels images for bounding box object detection.

What does RectLabel for object detection do?

With RectLabel, you can draw bounding boxes and annotate them, as well as drawing polygons and cubic beziers. It's easily customisable and should work well for most basic projects. It isn't complex to use, it has excellent user reviews,and the developers have provided clear directions. However, whilst very good for the price, it is quite limited in capability, and really does just do what it says on the box with no bells and whistles. Having said that, you can add some features with in-app purchasing, but depending on how much you need to add, you might want to just get a more complex programme in the first place.

Is RectLabel for object detection worth getting?

For a free app, RectLabel does its job well. If your project is very complicated, you might want to pay a bit more for an app with more features. Some users have reported issues with documentation, so bear that in mind if it could be an issue for you.