I am looking for very simple notepad that would allow "hierarchical" files editing. Something like TreePad but with much less bloat, more polish and possibly open sourced.
I don't know how you feel about wiki-like editing, but it sounds like you could use wikidPad.
Maybe you could try Evernote. It has notebooks and tags which can be tree organized (at least they could be in version 2), so maybe that's OK for you? It's not open source though and you actually have to pay for the more advanced features (like OCR-ing and indexing your pictures).
Another possibility is TreeSheets. It's not tree oriented in the same way as the program you posted, it's table based - each table cell can have a subtable, with which you then make a tree structure. It's one of those things you have to try to see if it suits you, but I find the concept very interesting. Also not open source, but it is freeware.
I'm afraid a good, lightweight tree editor does not exist. Almost all of these applications suffer from following problems:
- They use a "rich text editor". Because of this, you constantly need to keep layout in mind while taking notes and it is hard to use a uniform layout.
- They often use a proprietary file format. Wouldn't it be great if your tree notes are stored in HTML natively?
- They have only limited support for switching between recently used parts of the entire tree.
Because of these issues, I just use jEdit for now.
I use both Evernote and The Guide (http://theguide.sourceforge.net/index.html). The Guide is nice because although the file format is custom it is human readable and appears to be XML based. Evernote is nice if you intend to use the notes from multiple locations.
This is useful comparison: http://www.marktaw.com/reviews/Outliners.html
Use the "Lite" version. I don't know of any other tree outliner with similar feature set that can qualify as "much less bloat" than Treepad Lite, considering Treepad Lite is a portable app that is made up of one 587 KB executable and consumes less than 4 MB of RAM when initially opened. I've been using Treepad Lite for many many years, as far back as on my Intel Celeron PC, and it loads lightning fast even on that Celeron PC.
If being open source is also a requirement, then check out 2do:
I don't use it myself, but it appears to be quite bloat-free (I love Delphi programs because they are usually small and portable). However, it also appears to be very primitive, missing basic features like search.
You can try Zim: http://zim-wiki.org/.
It's very simple application with a multi-platform support. Your notes are saved as text files in folder hierarchies, so you can access them even without the application.