I've no idea if this is of interest, but... I've been on the hunt for a heiarchial notes app myself after having been used to the late-great old open-source Win prog, KeyNote, which like BasKet, is also unfortunately unique.
(My keynote thread)
http://ubuntuforums.org/showthread.php?p=3146207
And now I'm running the formerly blazing fast
KeyNote as a semisluggish WINE app until I can migrate to the next best thing I've found for the express purpose of being a tree-type writing app,
NoteCase. It's lacking the multi-tree tabs, but it's light, cross-platform and works as well on my xubuntu PC as the kubuntu PC. Similar and of good performance on my setup:
Gjots
Kjots (KDE)
Desktop wiki sorts:
Zim
Tomboy (Gnome)
Not-so-actively maintained:
Tuxcards
Newton
If you're into Java there's a couple Java note apps. They weren't my bag.
If you want a whole 'nother approach, the LaTeX-based
LyX can do a lot. If you use the multi-format attatchment capabilities of BasKet, nothing will match it. The closest thing may be
kdissert, a mind-mapping app?
Bookmarks