I also enjoyed Muse[1], which was made available in the iOS app store, and is now being maintained as "Allume"[2] (an unnamed company bought the Muse name).

Essentially a digital pinboard, it was an interesting experiment in "designed for tablet" UX and was developed far enough to be an app. The gesture based UX, nested board, PDF reader, and transclusion of PDF fragments are all interesting ideas.

[1]: https://www.inkandswitch.com/muse/ [2]: https://allume.com/