This is not a complaint. nGUI is great, but it’s also low-level - For example, instead of providing a scrolling listbox, it provides scripts you can use to make a scrolling listbox. And art assets are, reasonably, not it’s priority.
What helper packages do you commonly use for nGUI, and why?
I think you are very very very confused as to what the word ‘low-level’ means…
That’s an argument over definitions.
Instead of adding a listbox, with nGUI I’d add a scroll bar script and so on. That’s low-level compared to, say, Winforms, where I’d add a ListBox.
Now, if you have some confusion over what I actually meant with the question, I’d be glad to clear that up.
Edit: Although you did highlight a possible poor choice of words, so thank you.
The nGUI team is adding prefab support in v2.6.0. I very much doubt it was in response to my post, but thank you anyway. 
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It could be there are no real useful ones. But that, too, would be useful to know.