A data scientist friend of mine had some success with Figure8 but I haven't used it myself.
Honestly I always roll my own, it's dead fast to throw a simple GUI together in tkinter and it makes it easy to integrate your own models and custom sample rendering/plotting.
That is if you're doing simple discrete class labeling, as opposed to more complex labeling like box-labeling for image segmentation, or text2speech labeling for example.
