Haven't tried this yet but looks interesting:

"ultra_outliner provides an open and accessible canvas for adding and managing story elements as cards. With an intuitive panning and zooming workspace, writers have all the space they need to brainstorm and arrange their thoughts."

http://ultraoutliner.com/

The documentation page is worth checking out as well, they explain the concept behind the product:

http://ultraoutliner.com/documentation/page?path=concept.md&locale=en

And there is also a Youtube video:

https://youtu.be/p31I2I2Jshs

and some forum discussions about the development:

https://www.reddit.com/r/Screenwriting/comments/51xnls/outlining_with_cards/

http://www.indietalk.com/showthread.php?p=415653

http://www.writingforums.org/threads/outlining-with-cards.148252/

Not entirely unlike Scapple, by Literature and Latte (of Scrivener fame). Looks interesting, I shall give it a whirl.

Also reminds me of VUE and CmapTools

Interesting concept, documentation, video.. The beta itself is a bit rough. Teeeeeeeny tiny font sizes. Good start, though.

MadaboutDana wrote:

Not entirely unlike Scapple, by Literature and Latte (of Scrivener
fame). Looks interesting, I shall give it a whirl.

Supernotecard (https://www.supernotecard.com also aimed at writers, is another comparator - long-standing as an index-card tool. Thanks for the tip-off about ultra; I've downloaded it and will try it out.